Once you clear a level you can replay it two more times. Each time you clear it the level will be more difficult.
There are 3 tiers for each level, plus a replayable 4th one!
Sometimes hard levels will pop up on the map! They'll have a spinning red light and will be taller than the others. You'll get more rewards for completing hard levels.
Make sure to stock up on boosts! These levels are a bit more tricky.