If you want a short answer, it would be "to selfship yourself with a fictional character, usually your favourite". However, to yume isn't limited to romantic aspects, it can also be a platonic, queeplatonic, some even familial connections and it can be via OC or a self insert. I think it's such a personal experience that, in my opinion, a good definition would be "to create a meaningful connection with a character, directly or indirectly".