A heart can be given to many different things in LittleBigPlanet. It is an in-game bookmark so you can easily find the content that you liked again.
You can heart creators, levels and items in Popit. Hearting creators will cause their activities to appear in your LBP2 Activity Feed, hearting levels will add them to your Hearted Levels List and hearting items in Popit will add them to your Hearted page in the Personal section of Popit. (simply the Hearted page in LBP1).