A cover is a performance of a song that was originally released by someone else. The performer is not the song's original recording artist, and that is the point of the credit. When an upload carries the word "Cover", we read it as a plain statement of roles: one act is performing, and a different act is the one the song belongs to. It is a small label, but it settles a question that comes up whenever a song appears under a different act's name.
The credit sits in one of two places, the title or the description, and often in both. "Cover" or "Cover by" in a title marks the upload as a cover. An "Original song" or "Original" line in the description names whose song it is, and that second line answers the question a viewer actually has when they open the upload: whose song is this. The original may be named in parentheses, on its own line, or after a label such as Original. If the description is silent, the title may still mark it as a cover.
Covers turn up in a few places on official channels. One is a stage moment at a concert tour, where an act performs another act's song, and the title usually carries the concert or tour name. Others are a special segment on a channel, or a session recorded for a playlist-style series, where the series name is usually in the title. A remix reworks an existing recording, a re-recording is the original act performing its own song again, and a language version is the original act singing its own song in another language; in a cover, a different act performs someone else's song.
The original credit matters because it points you to the original release. Once you know whose song it is, you can look up that song and the act behind it, and you can keep the two acts' names straight when both appear in one title. A title that names two acts can be confusing at a glance, since one is performing and the other is the song's original act, and the credit lines tell you which is which. A cover credit tells you whose song it is. It says nothing about the circumstances of the performance, and credit lines are written differently from channel to channel.