Energy is used to cut down Trees and Seaweed to clear land, and to tap Dolphins and Whales for prizes.  Periodically there are other items to tap, such as Jak-o-Lanterns or Turkeys, which are introduced for a specific period or event.

Energy regenerates at a rate of 1 unit every 5 minutes but will not increase past 15 units, the maximum amount you can have.  (NB. Android users have a maximum of xx units.)