When a system exists at some temperature, the energy of particles' degrees of freedom are randomly distributed according to a Boltzmann distribution. The energy of such systems is not constant; it's constantly fluctuating due to exchange of energy with surroundings. The Andersen thermostat models this exchange of energy with the surroundings as random perturbations on randomly selected particles. This is in contrast to methods such as the Nosé–Hoover thermostat.