They have to work together to collect the hearts on each level to feed their tree. The trouble is, these levels aren't just any levels, each one is broken up into comic-book pane windows, but then shuffled out of order. So stepping from the bottom left pane could see you appear in the top middle one, plummeting to your doom in the process.
Working out the order of the panes isn't too tough, but figuring out how to catch the hearts is, in some levels your creatures can stack up to reach them or transform into other animals with particular powers, including the ability to shuffle the panes around.
Figuring all this out is maddeningly fun, but the game's eight bit-aesthetic, gentle charm and subtle tones keep you from getting too wound up. Hopefully the challenge will continue to vary and bring up new ways to play in the later levels.