Why scientists care
Enthalpy is a thermodynamic state function. At constant pressure, under the usual conditions for pressure-volume work, its change is especially useful for discussing heat transferred.
A negative reaction enthalpy is associated with an exothermic process under those conditions; a positive value with an endothermic one.
A concrete example
Combustion releases energy to the surroundings; at constant pressure this is described by a negative enthalpy change.
Try to notice
Science becomes memorable when a definition is connected to something measurable or observable.