This statement, which has become proverbial, is part of the Gospel (Jn. 8:31-42) of today's Mass (Wednesday of 5th Week of Lent). But most people are perhaps unaware of it's context: who said it, to whom and the necessary condition on which it is based?
Christ is the one who said it and He was speaking "to the Jews who had come to believe in Him." It is a conditional statement: "If you abide in my word, you shall truly be my disciples, and you shall know the truth, and the truth shall set you free."
In order for you to have freedom, therefore, you need to 1) know the truth. The truth can make you free only if you know it. And 2) you will know the truth if you are a true disciple of Jesus Christ (He being the Way, the Truth and the Life: noone comes to the Father except through Him). And 3) you are a true disciple of Christ if you abide in His word
This is the proper context of "the truth shall set you free." Get with Jesus and you can live, move, breathe, be for real, deeply, thoroughly and meaningfully.
There is an essentially ecclesial dimension also to this abiding. The place of the Lord's abiding is with the Father and with the body of believers: the Catholic and Apostolic Church. Just two chapters earlier in the same Gospel of John (6:57) we read "He who eats my flesh, and drink my blood abides in me and I in him." Holy Communion! Catholic Mass! That is the way to Christian abiding communion with the eternal Father in the eternal Word made Flesh.
So the question of freedom comes down to the fundamental problem of abiding with God in Christ. No Christ, no freedom. No Church (no Mass), no Christ. The foundation for the source of freedom is the truth which Christ teaches each man through the sacramental ministry of His Church.