This is a clearance sac, which means white sacrificed the knight to clear space on the kingside for his other pieces to come in and attack you. Your opponent was probably trying to get rid of the kingside pawns so he could use his Queen and rooks to attack the open files, but he didn't see that your bishop could attack his Queen.
Why is this brilliant?
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My opponent played this and I don't see how this is brilliant. (plz don't judge if it's obvious and I'm just not seeing it)
Nxg5 is brilliant