Upper strut brace from the Mk1 Audi TT. Basically metal tube that connects the two strut towers together in the engine bay. It's standard fit from the factory on the Audi TT but not on the golf. Straight fit apart from you need to drill 3 holes in each of your strut towers to fit it.
Benefits are it tightens the front end up and prevents body roll/ flex. I had one on my old mk3 but not fitted one to the mk4 yet, I'm a bit bothered about drilling the holes but that's just me being precious lol.