Hero Power shows how upgraded a hero is. Each enhancement increases a hero's power. You can find the current power of a hero in the bottom-right corner of the hero window:
You can also view your Total Hero Power. To do it, press the Heroes button:
It is at the top of the menu:
This number represents the highest total power your heroes have ever reached on this game profile. It does not show the current power of all your heroes.
Note
Total Hero Power can only be increased. Resetting Gift of the Elements will not change the number, since the game saves the maximum achieved value.