According to the kinetic molecular theory, particles in a gaseous state, whether in the form of molecules or atomic gases, are in constant, straight-line motion, occasionally bouncing off each other in perfectly elastic collisions which change their direction without slowing them down. It depends on a number of assumptions, such as the idea that the particles are very small compared to the space between them.
Other assumptions of the kinetic molecular theory are that the particles of a gas have no forces that attract them to other particles or to their container. It also assumes that the average kinetic energy of the gas particles rely on temperature alone. All of these assumptions, and the kinetic molecular theory itself, are an explanation of the observed behavior of gases, in particular the relationship of temperature, volume, pressure and the number of gas particles in a container.
For instance, the pressure on the inside of a container from a contained gas is, according to this theory, the result of particles impacting the container and exerting force on it. The more particles there are, the more force they exert overall. Similarly, pressure increases with temperature, as this increases the speed of the particles, both increasing their force and the rate at which they impact the container.