Completing a mission with three stars unlocks a challenge mode, which puts even the most experienced players to the test. Missions completed in challenge mode will earn you an additional star.
You can earn between one and three stars after the successful completion of a single-player mission. Easy mode will get you silver stars, while playing the game in normal mode gets you gold stars. The sum of stars a campaign mission may grant, can never be more than three regardless of their colour. Stars can be used as ”in-game“ currency to unlock special skills as the campaigns progresses.