You know those manual hand pumps you use to inflate basketballs, soccer bals and such. So there is a tube and on one end the handle and the other a “needle” you stick into the ball. OK so you push down the handle and put air into the ball. Then you pull the handle and presumably pul air into the tube. But how does that air get into the tube? Clearly it is not from the ball , it comes from the surrounding air, but how is it getting in?
The piston has a seal that was made of leather on old bicycle pumps. It is pushed by a metal washer and has the edges folded in the opposite direction. This allows the edge to act as a check valve. When the pistonis pulled air flows in the gap between the rod and end cap and between the edge of the leather and the inside of the tube. When the rod is pushed air pressure pushes the edge of the leather against the side of the tube sealing it.
Here’s a diagram of a simple check valve. Air will flow freely from left to right, but not right to left.