It is the boltzmann equation of kinetic theory. It is a differential equation for a 6 dimensional function f(r,v) which describes the number of particles at location r, with velocity v. This distribution changes as the particles fly through space and collide with one another. It has important applications in the theory of hot dilute gases (e.g. rocket engines) and the theory of conductivity in solid-state physics. There are already practical approximations and numerical methods used by physicists who need practical answers to these problems so I think this is mostly of interest to mathematicians...