The feature was released only yesterday (November 17, 2020), but we've already learned almost everything there is to know about what you can do with Fleets.
They're little posts that stay up for 24 hours, usually consisting of an image or video with some additional text, emojis, or stickers. Twitter's Fleets feature is their own take on the Stories you've seen on Snapchat, Instagram, Facebook, and even LinkedIn.