As you may already know, Splot’s task is to help Nettle the fairy. Nettle has had an argument with her mother, resulting in her making all the stars fall from the sky (or so she thinks!).
In the game you are supposed to collect golden stars and return them to the heavens. Each new star ups your overall score, and when you have collected enough of them, you can obtain new characters. You can also combo when collecting stars and this gives you a better multiplier and thus more stars.
Golden stars are floating in the air at the beginning of each level. When you collect them they become silver stars, so if you want to play a level again you can easily see which stars have been picked up already. Silver stars can be collected to get better high scores in a specific level.