The molecules in a gas move quickly in random directions. They are far apart, so they move freely in straight lines until they collide with each other or with the walls of their container. The molecules have kinetic energy because they are moving.
The temperature of a gas is linked to the average kinetic energy of its molecules. When a gas is heated, the molecules gain kinetic energy and move faster on average. When a gas is cooled, they move more slowly on average.
Gas pressure is caused by molecules colliding with the walls of the container. Each collision exerts a tiny force on the wall. Billions of collisions every second add up to a steady force on each unit of area, and this is the pressure.
In a closed system, no gas can enter or leave, so the number of molecules stays the same. If the gas is heated, the faster molecules hit the walls harder and more often, so the pressure goes up. If the gas is cooled, the molecules hit the walls less hard and less often, so the pressure goes down.
