Kits are the cards you receive for earning stars when playing a puzzle. There are two types of kits, the ones you need to build the scenes. They look like this:
And the ones you can combine to create Boosters, Power-ups or Coins in the Tinker Table. They have a character on them:
You will also receive one character kit every time you unlock a new scene.

The Tinker Table is the place in the game where you can see the kits that you have collected (in the Kits tab) and also use them to create Boosters or Power-ups (in the Items tab).

A notification on the Tinker Table icon (the second button on the level list view ) pops up when you have new opportunities to create useful Boosters or Power-ups. Don't miss them out!