The cylinder of a bicycle pump contains air. Its exit is sealed with a pressure gauge.
As the plunger is pushed in, the air is forced into a smaller volume. At the same time, the pressure gauge shows an increase in pressure.
It is this increase in pressure that allows the pump to do its work.
When the plunger is pulled out, the volume occupied by the gas grows larger, and the pressure drops.
Larger volume, smaller pressure
Smaller volume, higher pressure.
Pressure and volume are in what is called an inverse relation. As one rises, the other falls.
