Hello Максим! I agree with the other answers but I would say that the abbreviation "feat." can be used for referring to a "guest artist" at a performance or in the recording of a song or video. It could be a musician playing an instrument or it could be a vocalist singing with the main artist or group. Very often this appearance is by a relatively unknown artist and they will make a debut performance by being "featured" performing with the main artist or group. Alternatively a relatively unknown artist or group will have a very well-known artist "feature" in their performance to create some extra exposure for them. Some artists will also keep their "feat." artist a surprise at a live performance or concert until that particular song or presentation is performed. I hope that helps!

It is an abbreviation for "featuring." For example if Pat recorded a song, but Pat's friend Robin sang a single verse, then the song might be by "Pat featuring Robin" (or, abbreviated "Pat feat. Robin.") That way, people know that Robin contributed to the song. On the youtube videos you're seeing, they're saying "this other youtuber is going to show up."

Yes, it's an abbreviation for "featuring", meaning that some other artist is included in that music track. Typically that other artist has also a singing part in that song, so that's why he is "feat(ured)".