When a material melts, it absorbs energy without its temperature rising as quickly as it would during ordinary heating. When it solidifies, it gives that stored energy back. Engineers call this latent heat storage.
Energy hidden in a change of state
A phase-change material can buffer temperature swings in a building or thermal system if its melting point sits near the desired operating range. It needs a way for heat to enter and leave, and often a container so the liquid phase stays where it belongs. Choice of material affects capacity, cost and cycling behavior.
A useful buffer, not free cooling
The material does not create cold or eliminate heat. It merely shifts when heat is absorbed or released; eventually it must reset by cooling and solidifying. Slow heat transfer, leakage, supercooling or repeated-cycle degradation can limit a design. The intended temperature range is as important as the material’s headline storage capacity.
Related: how a heat pipe moves heat without a pump.
Source
US Department of Energy: Thermal Energy Storage with Phase-Change Materials.