I don’t understand why chess.com says this is a “Brilliant Move”? Can someone explain please?
Whites bishop on g5 is hanging, after Bxg5 Nxg5 Qxg5 black is up a piece. However this would remove the defense of the c7 pawn allowing for Nxc7 by white forking the king and the rook on a1 leading to the winning of the exchange. Chess.com grants brilliancies when a move loses material but either maintains or creates an advantage.
The move 8. e3 moves the pawn from e2 and allows the queen a clearer path to black's (undefended) knight on h5. Piggy-backing of Bansel's analysis 8... Bxg5 9. Nxg5 Qxg5, black is now open to the Nxc7 fork. However, if black doesn't play 9... Qxg5, the knight on h5 is now hanging because the queen on d1 has a clear path to h5. Even worse for black, That h5 knight has no square to retreat to, in part to the f4 square being defended by our hero pawn on e3.
In short, 8. e3 indirectly threatens the h5 knight and forces black to either concede it, or give up the fork/exchange.