As a gas is cooled, its particles move more and more slowly, and their kinetic energy falls. If a gas could be cooled enough, its pressure would fall towards zero, because the particles would hardly move or collide with the walls.
There is a lowest temperature that anything can reach. It is called absolute zero and it is -273 °C, or -273 degrees Celsius. At absolute zero the particles have the least possible energy. In the simple particle model used at GCSE, they have no movement at all, so they have no kinetic energy.
Nothing can be cooled below absolute zero, because the particles cannot move any less than not at all. In practice, scientists can get very close to absolute zero but never quite reach it.