To depressurize is to let up on the force of something, especially a liquid or a gas. An astronaut must make sure that her protective spacesuit doesn't depressurize when she's performing a space walk.
Depressurize means to slow or completely stop some kind of pressure, and this verb is almost always used to describe a halt in the flow of air, oxygen, or a liquid of some kind. In movies, when a plane or spacecraft depressurizes, there's a sudden, dangerous drop in breathable air. On a commercial airplane, individual oxygen masks are available in case of such an event. The Latin root of depressurize means "to press."