This is a descriptive term for an exothermic reaction (a chemical process that releases heat). The polymerisation process is an exothermic reaction. This is unlikely to be felt when using liquid and powder as the rate of polymerisation is controlled by the initiator (in the polymer powder) and the catalyst (in the monomer liquid). But with UV cured gels there can be several situations where the polymerisation produces a lot of heat that can be felt by the nail bed. This can be quite painful and can even burn the nail bed.
This can happen if the wrong UV lamp is used, the coating is too thick or the nail plate is thin. It is a situation that should be avoided by using a matched system, thin UV gel coatings, avoiding damage to the nail plate or being aware that a nail plate is naturally thin.