If the end of a bicycle pump is covered by your thumb and the plunger is pressed, you can feel the increase in pressure of the gas inside.This is because the volume of the gas has reduced.
The trapped gas particles inside the pump now have less distance to travel. This means that they hit the walls of the container more frequently, causing this increase in pressure. Note that the temperature of the gas has not changed.
Similarly, if the volume of a gas is increased, without a change in temperature, then the pressure of the gas is reduced, because the gas particles have further to move before hitting the walls of the container.
As the volume of the gas increases, the pressure of the gas decreases, when the gas temperature is constant.