Energy is a gameplay resource used by the Scoundrel, Sniper, Operative and Gunslinger combat styles (former Imperial Agent and Smuggler combat styles).
Energy starts at 100% and regenerates after use. Energy is used by most of their damaging abilities apart from Rifle Shot/Flurry of Bolts. If energy gets too low Stim Boost/Pugnacity or Adrenaline Probe/Cool Head can be used. Energy regenerates fastest above 60 or so. Out of combat Recuperate can restore energy faster.