How to understand the "best move"


At the top left of your screen, you have the move that was played (in this case Nxd4). The best move is what the engine thinks you should have played instead of Nxd4 on that turn.
Now that we’ve cleared that up, you can see that Bh2+ is a legal move for black, if they played it instead of Nxd4.
I hope this helped, let me know if it didn’t and I can try to explain it a different way