When a fixed sample of gas undergoes a temperature change, the resulting kinetic energy changes as well.
For example, when a fixed sample of gas is heated up, the kinetic energy increases and the gas molecules begin to move faster within the container they are being held, bouncing off of the container with a larger force and more speed, which in turn, increases the pressure as well.
If a fixed sample of gas is cooled, the exact opposite happens. The gas molecules begin to slow down because there is less resulting kinetic energy and they don't hit the walls of the container with as much force or as frequently, which results in a lesser pressure. That is why balloons will shrink when placed in the cold.
I hope this helped! :))
