An "Asset Flip" is a commercial game puzzled together from ready-made bits as cheap as possible (e.g. get some shooter template code, drop in some free/cheap character and weapon models, copy or make some really basic levels), without consideration for the quality of the resulting game.

If you've ever followed a Hello World game building tutorial for UE or Unity, somebody has taken the end result of that tutorial and placed it on a game marketplace somewhere.

It was/is a bit taboo, but the gaming phenomena PUBG also used UE marketplace assets, although they added value by making an enjoyable game around it.

I've generally heard the term used to refer to low-effort games that are basically just third-party assets packaged up and sold as a full experience, while having little to no "playable content". The availability of these assets could encourage this since they are freely available.