Energy is the primary resource to use abilites in Starcraft 2. Most hero abilites cost a certain amount of energy to be used. All heroes start with roughly 100 energy and gain additional maximum energy and energy regenration as they level up.
Energy reserves and regeneration can be increased through talents, e.g. Power Crystals or Masteries, e.g. Energy Mastery. However, some talents or Masteries will also incur penalties in exchange for powerful bonuses such as Ionized Chambers talent, Psionic Warrior Mastery or Pierce the Veil Mastery.