A freeze-out (in cosmology) is the occurrence of some type of particle existing long-term if the processes that destroy it fall off, which can happen after the particle's decoupling. The left-over particles that are termed relics. A freeze-out left many of the neutrinos in the universe today, but the same can be said for virtually any particle that has been around. The term freeze-out grows out of the fact that they occurred when the temperature fell (and the density fell) as the universe expanded (Hubble expansion).
Relic abundances from freeze-outs can be calculated using observed and calculated reaction rates of particles (particle physics) and working backward, extrapolating what would happen as the universe was growing from being very small. Abundances observed subsequently that match that of these calculations are a major contribution to the confirmation of the Big Bang theory.