Exothermicity is the release of heat during the chemical reaction of a mixture of reagents. This heat release is at a maximum at the exothermic peak. It can be very great.
Thermosetting resins are all exothermic and some of them very much so. It is therefore very important to have good knowledge of the reaction process (onset, end, total energy released) for each resin used in order to correctly control the reaction.
For example, for large components (or those locally very thick) weakly exothermic resins will be preferred and they will be cured in a thermal cycle with temperature segments in which the reaction speed is low.
Exothermicity is generally favoured by high temperature rises that make most products more reactive.