The molecules in a gas move randomly at high speed in all directions. As they move they keep colliding with each other and with the walls of their container. Every time a molecule hits a wall it pushes on the wall with a tiny force. There are billions of collisions every second, so together they produce a steady force on each square metre of wall. This is the pressure of the gas.
The temperature of a gas depends on the average kinetic energy of its molecules. At a higher temperature the molecules move faster, so they have more kinetic energy.
Now think about a closed system, such as a sealed container with a fixed volume and a fixed number of molecules. When it is heated the molecules speed up. They hit the walls more often and with more force each time, so the pressure increases. If the gas is cooled, the molecules slow down and the pressure decreases.
