Outfits are vanity items that can be unlocked by collecting mystery boxes. Once an outfit is unlocked, it can be put on the character it belongs to in the treehouse building. There are 24 outfits in total, three for every character. Each outfit also comes with a host of different colors.
"So if you weren't satisfied with a horse wearing a red bow tie, green is now an option." - kylerk, game devcharacters have three outfit slots, others only have two and two different outfits in one of them.