What is heat of reaction?

Heat of reaction is ,if I simply put, the heat which is generated or absorbed with chemical reactions.
For example, the heats are generated when we generate ammonia with nitrogen and hydrogen. This is heat of reaction.
Chemical formula　at that time is "N₂ + 3H₂ = 2NH₃ + 91.80 kJ". This 91.80KJ is heat of reaction.

Heat of reaction is used in many fields too.
For example,it is Hiyaron that is used in summer. Hiyaron causes an endoergic reaction that melts ammonium nitrate and urea being mixed with cold water, and taking surrounding fever.