You can see the almost exclusively here the black bishop moves to b4 to check the white king and white has a decision to make.
Bd2 seems to be the most popular move and gives white a small advantage in the engines. I think because the black bishop now has nowhere to go so has to exchange bishops and white can then develop the b1 knight.
But Nc3 is known as the Greco attack and has a great trap which is well worth looking at so we will. What do you play in this position?
If you moved the b knight to d2 then d5 would be coming and black looks better.
Of course in this position black is going to take the e4 pawn with the knight for free so it’s pretty much the only move for black.
To unpin the white king white is going to castle. Now black has two attackers on the c3 knight so the main line is Bxc3 which leads us to the end of the main line for the Greco attack.
From here the most popular move is d5 which takes us into the Moeller attack so that’s for another post! Bxc3 from white invites black to take back with the knight by Nxc3 but that would be a mistake. As you can see in the checkmate post white would move the queen to b3 attacking the knight and once the knight moves the bishop is set loose with Bxf7+!
At this point the black king has to move to f8 and has lost the chance to castle and is about to lose their queen so white is clearly winning which is why black should have played d5 earlier to block the bishop when it wasn’t defended by the queen. Even after taking the c3 pawn white should still play d5 as if the white bishop takes the d5 pawn the c3 black knight can take back.