As a gas is cooled, its particles move more slowly and have less kinetic energy. If you keep cooling, there is a point where the particles have as little energy as it is possible to have. This is called absolute zero, and it is the lowest temperature that can exist.
Absolute zero is -273 °C. At this temperature the particles have almost no kinetic energy and have effectively stopped moving. Nothing can be colder, because particles cannot move any less than not at all.
This also fits with gas pressure. Pressure is caused by moving particles hitting the walls of a container. At absolute zero the particles are not moving, so there are no collisions with the walls and the pressure of the gas would be zero.