It isn't that there is pressure on an imaginary square. It is just that the enemy King is on c8 (probably from queenside castling) and when people castle on opposite sides of the board, usually it is beneficial to either pawn storm, or to attack the enemy king with pieces such as half open files for rooks. The computer line sacrifices the c2 pawn, so that the a1 rook can go to c1 and put pressure on the enemy king which is castled.
If you play the bishop to e3, then the knight can capture on e3 and then queen captures your g5 knight (like in the game).
Here are some videos of mine which might help you 
I don't know what to call it, but I run into it often. I don't see indirect threats. From what I gather after looking at the game with Stockfish, white should have played Bd2 instead of Be3 because Nxc2 then attacks the e3 square. Is there any other reason why Be3 shouldn't have been played in this game? The move is on 17.