An instance is a timed quest area that presents players with a set of parameters that must be met before lockout. Once the timer is up and the instance is locked, you cannot enter it again until after server reset. (Midnight EST)
SWG Legends features the six main heroic instances from Live - Tusken King (TK), Exar Kun (EK), Imperial Star Destroyer (ISD), Axkva Min (AM), IG-88 Droid Factory (IG), and Echo Base (Hoth), which is instanced separately for both Rebel and Imperial factions.
Heroic instances ("heroics" for short) are the main end-game content for SWG. Each heroic, minus Hoth, has a quest prerequisite that you must complete before you are keyed to enter. Once you have completed the prequests, you can enter the instances through each of their physical locations. A full group of eight players is advised for entering these areas, as some of the enemies within are much more difficult than your typical gold elites.
The only requirement for entering the Hoth instance is to be either Rebel or Imperial.
The reward for completing heroics, along with any loot that you might have obtained from NPCs inside, are heroic tokens associated with that instance. These tokens are used to purchase heroic jewelry, buff items, armor pieces, and weapons, some of which are among the best items in the game.