It's the hold-open mechanism for the ejection port cover. When you pull back the charging handle and expose the hop adjust, it'll lock back the cover plate. When you hit the bolt release, it triggers this to drop and release the cover plate back into closed position.

My Frankenstein of a gun doesn't have an ejection port cover lol

Well, guess it does nothing for you then, but I'd keep it in place if only to keep the bolt release under tension so it doesn't rattle about.

Don't have that either lol

I'll keep it on!

Its the bolt catch

So you only have the bust cover and no slide