Wind chest

From Ccpi

A wind chest, or windchest, is a wooden box that holds air to be delivered to organ pipes, which are attached to the top. During a stop, air flows from a reservoir into the windchest, where it is stored until notes are played. When a note is played, a valve block opens, allowing air to flow into the organ pipe. As soon as air enters the pipe, it comes into contact with the edge at the front, and vibrates. Those waves that fit exactly within the pipe are amplified through resonance.

Wind chests work in the same way that lungs provide air to move across vocal chords.