Writers frequently refer to the liberty in this passage as "Gospel Liberty". Primarily, this is because the gospel is the vehicle for it's proclamation. Secondarily, special terminology is helpful because it lessens the possibility of confusing it with the very different subject of freedom/liberty of the will.

Gospel liberty is predicated upon at least two conditions; (1.) an inward spiritual condition (regeneration) which enables one to will as he/she ought, and (2.) such a relationship with God (adoption) so as to be delivered from the hindering constraints of fear and brought under the noble motivation of love. Gal. 4:6; Ep.1:13-14; Ro. 8:14-17

As an apostle of Christ, Paul uses his authority to instruct the Galatians by command. The course they are commanded to follow is sharply delineated by a positive and a negative. Positively he says, "Stand fast in this liberty". The words "stand fast" indicate firm constancy as opposed to vacillating indecision, passivity, or compromise. In other words, "keep on keeping on." A.T. Robertson translates it as "keep on standing.

Negatively, Paul says, "Don't be snared by the yoke of bondage.” There are various forms of bondage that people may be snared into. From contextual considerations one may safely conclude that the specific form of bondage Paul warns against is legalism. This is supported by the following:

1. His reference to, and analogy with the earlier trouble at the Antioch church by Judaizers. [ch. 2:4 & ch.]
2. The similarity of the issues with those in the Antioch debate. [ch. 3:1-5, 11, & ch.; 5:2-4; Acts 15:1, 5 & ch.]
3. The parallel use of descriptive terminology in both cases (yoke of bondage, circumcision, law keeping). [Gal.5:1-4; Acts 15:5,10]
4. The common remedy for the error. [Gal. 5:6 ; 6:14-16 ; Acts 10:11, 24]
5. The yoke of legalism obtains when Judaism displaces gospel teaching. [Gal. 1:13-14; 2:4; 3:1-2; 4:9-11; 6:12,13]

GOSPEL LIBERTY IS SPECIAL AND UNIQUE
Paul's language makes it clear this liberty is special. The definite article "the" distinguishes it from other types or forms. He says, "Stand fast therefore in 'the' liberty wherewith Christ hath made us free." This is true liberty and wholly unlike the distortions proposed by free-thinking secular philosophers and free-spirited antinomians. Such as these propose freedom from divine authority but it ends miserably with bondage to sinful-self. On the other hand, the liberty Paul contends for is grounded in divine deliverance from the condemnation and curse of sin, and is experientially realized by the sending of the Spirit of adoption, whereby we cry Abba, Father (Gal. 4:4-7; Ro. 8:15).

Furthermore, Paul argues this Liberty is special because:
1. It is the only lawful freedom approved and promised of God (Gal. 3:14; 6:15-16);
2. Therein is deliverance from this evil world (Gal. 1:4).
3. Only therein is there freedom from the yoke, the curse, and the condemnation of the law (5:16, 18).
4. It is a fulfillment of the blessing of Abraham to his heirs (3:14; 4:22,23).
5. Christ is the efficient cause of it, according to God's will (5:1; 1:4).
6. The will of God is the ground of it, as expressed in the New Covenant (1:4; 3:17; 4:24,26,27).
7. The New Covenant children are, as Isaac, the heirs of it (4:28, 31).
8. The heirs of promise obtain possession of it by the Holy Spirit (3:14; 4:6; 5:18, 22-25).
9. The parameters of this liberty are exceedingly broad and involve values and virtues which embrace all areas and circumstances of human life (Gal. 5:16, 22-25: 6:1, 2 & ch).

LIBERTY VS BONDAGE
Paul's statement (Gal. 5:1) places liberty and bondage as diametric opposites; there is no middle ground. A slave may be emancipated from his bondage and become a free man, but he cannot be both free and in bondage at the same time and in the same way. Hence Paul's statement in verse four: "Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace." (Gal. 5: 4)
Although these had once enjoyed liberty they were now under the yoke of bondage. Furthermore, Paul's explicit command to the Galatians to "stand fast" clearly implies that they who are negligent cannot keep nor enjoy this liberty. Thus, gospel liberty is conditioned on steadfastness. They who live and handle gospel truth carelessly are therefore deluded if they assume they are abiding in the liberty "wherewith Christ hath made us free."

Although Paul's main theme in this epistle addresses the bondage yoke of Judaism, there are other forms of slavery which oppose gospel liberty. Paul alludes to several other forms in Galatians 5:19-21.
Please read the passage for yourself. For clarity some have divided his list into three categories; I prefer four.
They are as follow:

1. Slavery to Sensual Self --- [adultery, fornication, uncleanness, lasciviousness]
Sexual activity outside the covenant of marriage head this category. Also included in this category is pornography of whatever sort, whether it be literature, photo images, videos, and television/motion-picture programming where explicit and ungodly language and sexual relationships are depicted.

2. Slavery to Idolatrous Self --- [idolatry, witchcraft]
Includes false worship of all kinds whether ancient paganism or the modem variety. In the latter, false values of worth and importance are attributed to either “ideas” or physical and material objects. Any false idea or concept which turns one from truth and worship of the Creator is idolatrous (Ro. 1: 21-25; 2 Cor. 10: 4-5). Hence, Paul rightly calls covetousness Idolatry because such a one is under the power of a false concept which distorts the true purpose and value of material possessions (Ephesians 5:5; Col. 3:5; Matt. 6:24).

3. Slavery to Egotistical Self --- [hatred, variance, emulations, wrath, strife, seditions, heresies, envyings, murders]
This category includes a comprehensive list of words which describe the emotions, attitudes, and actions commonly exhibited by those seeking to gain personal or party advantage. In these ways self-love prompts hatred and hostility, in it's many and varied manifestations, toward those who get in the way of personal desires and ambition.

4. Slavery to Hedonistic Self --- [drunkenness, revelings, & such like (partying)]
Hedonism is the doctrine that pleasure and happiness is the highest obtainable good. Paul refers to those who practice this as "lovers of pleasure" (2 Ti. 3:4). This has serious ethical ramifications because the quest for pleasure frequently infringes on moral obligations and results in compromise or neglect of the same. Please notice that this is one of the specific characteristics of the social environment during the last days (2 Ti. 3:1-7).

SUMMARY
There are many forms of bondage to be aware of and avoid. The enemies of truth have also proposed counterfeit forms of Liberty which are false. There is but one true Liberty “wherewith Christ hath made us free.” Let us stand fast in it.

"For brethren, ye have been called unto liberty; only use not liberty, for an occasion to the flesh, but by love serve one another." Gal. 5:13