Page "Enthalpy" Paragraph 4

For quasistatic processes under constant pressure, ΔH is equal to the change in the internal energy of the system, plus the work that the system has done on its surroundings.

This means that the change in enthalpy under such conditions is the heat absorbed ( or released ) by a chemical reaction.

Enthalpies for chemical substances at constant pressure assume standard state ; exactly 25 ° C at 1 atmospheric pressure.