The first Creator Beta for Dreams was held last month but it was exclusively for players on Media Molecule’s mailing list. The upcoming beta lets players try out the creator mode. You could in theory make your own game just as you could with Little Big Planet but you could also make music and presumably even little movies with the puppets you make in this mode.
The fine folks over at Media Molecule held a livestream event earlier today to show off some of the creator tools you will be able to use in Dreams. If you would like to watch that video then you can check it out on their Twitch channel.