Enthalpy is the chemical potential energy in a system. Endothermic reactions are when energy goes into the system, increasing the enthalpy. Exothermic reactions are when energy leaves the system, decreasing the enthalpy. $\Delta H^{\theta }=\sum bondsbroken+\sum bondsformed$

Reaction occurs to reduce enthalpy. Standard enthalpy change ($\Delta H^{\theta }$) is the measure of energy change of the system, that occurs in a chemical equation, per mol. Standard conditions, so 100 kPa and 25 $C°$

Bond enthalpy

The energy required to break one mole of bonds in a gaseous molecule under standard conditions.

Average bond enthalpy takes the average value of the bond enthalpy of the same bonds in different environments. For example, the two H-O bonds in $H_{2}O$ require different amounts of energy to break. The average is the average bond enthalpy of H-O.