So in thermal physics we assume that these motions are random, and we use the laws of probability to predict how the material as a whole ought to behave. We can't possibly follow every detail of the motions of so many particles.
Examples include the air in a balloon, the water in a lake, the electrons in a chunk of metal, and the photons given off by the sun.
Thermal physics deals with collections of large numbers of particles-typically 10 23 or so.
