Each level offers between 1 - 3 stars whenever you complete it with a minumum of one unit alive, with bigger rewards for completing objectives and earning these three stars. Don't forget though that you choose to don't be required to meet the objectives while doing so.
For example, if the objectives are “Use only basic abilities,” “Survive with 2 allies,” and “Win with 75% health” and you get the first two stars on your first playthrough, you don't still have to only use basic abilities and survive with 2 allies intact on your next playthrough. Just win with 75% health and your third star will unlock.
