TL; DR Over time, things tend to change so that everything is the same temperature.
Thermalization is the process of physical bodies, generally subatomic particles, reaching thermal equilibrium through mutual interaction. In common parlance this is often referred to simply as entropy, although entropy, AKA thermodynamic equilibrium, applies to not just thermal equilibrium but also mechanical equilibrium, radiative equilibrium, and chemical equilibrium.
Thermalization is most often used when talking about plasma or nuclear reactions, as these are systems in which there are likely to be types of significant imbalance in play in addition to thermal energy. In some cases, thermal equilibrium will encourage other sorts of reactions, such as nuclear reactions.
It is also common to see the term 'thermalize', meaning to cause a particle to change its thermal energy to match its environment, as in "the cold water quickly thermalizes the high-energy neutrons".