Loose literally means not fastened, restrained, contained, fixed, or rigid. A secondary understanding is to release from obligation or to absolve.
In Bible times, a yoke was essentially a Rabbi‘s teaching of what was prohibited and what was allowed, according to his interpretation of Scripture. The process of determining what things were allowed was the act of “loosing.” To “loose” something was to allow it, that is, to let it go. To “loose” implies freedom.
Since Jesus‘ yoke was light, there must have been many things that he had “loosed.”
Contrast to bind.