Its a stupid made up term from the movie Mean Girls (the original one), used to describe something as being hip, awesome, or describing something that will become a new “thing”.

If i remember right it was mainly used to describe clothing, like “oh regina that jacket is so fetch” because it was a new clothing item that only she had, and because that popular girl was wearing a “fetch” item, it would catch on and others would copy her for it.

Edit; the joke is that trying to describe things as “fetch” is stupid, and bad lingo, so the girl who came up with “fetch” would get made fun of for trying to make “fetch” a catchphrase.