A kind of DM note that is unsigned. To prove its you who sent it, it uses a wrapping (other notes) that dont have the message but have the signature.
So dividing these notes, it is like message itself can't be proved to be yours, unless you also have the wrapping, and because of that there is a plausible deniability. 

More here: 
https://github.com/nostr-protocol/nips/blob/master/59.md