After receiving the Ten Commandments, the mishpatim (ordinances) were given. The first ordinance given is the designation of freedom a Jewish servant receives after only serving a for a limited amount of time. It is as if the Torah is saying, that the Jewish people are not meant to remain in bondage again, not even as indentured servants.
The only exception appears to be the servant, who after six years, would prefer to remain with his master. He declines his freedom; subsequently, his ear is pierced by an awl on a door to mark his perpetual servitude. This act serves as a reminder that the same ear that was pierced, should have heeded the call to freedom. Yet, according to some commentators, even he is released from bondage upon the arrival of the Jubilee year.
Symbolically, the door represents freedom, because of the blood of the Pesach offering that was placed on the doorposts in Egypt, right before B’nei Yisrael was freed. Ultimately, our actual freedom is through Torah itself. As explained in the following manner:
The commandments were inscribed (cherut) on stone tablets; yet, the Hebrew word cherut, with a different vowelization, means “freedom.” What is the connection? When we observe the commandments of G-d, we are freed from slavery to our yetzer harah (evil inclination).