You use energy when collecting rent from buildings and building any structure (by clicking at each stage in its construction). Energy is replenished automatically. You get one unit of energy every five minutes. When you reach a new level, all your energy is replenished. Visiting friends gives you +1 energy.
You can also ask your friends for +3 energy. When your energy is low, a pop-up button will appear. Click “Ask friends” and select a friend to ask him/her for bonus energy or purchase a refill with superbucks.