Discovered attack
A discovered attack happens when you move one piece out of the way and uncover an attack from a bishop, rook, or queen behind it.
The piece that moves can go anywhere useful: a capture, a second attack, or a quiet square. The damage is done by the piece that was hiding.
The bishop on a2 is aimed at the queen on g8, with the knight sitting on the diagonal. Any knight move uncovers the bishop. Ne7 is one way to step off that line and leave the queen hanging.
Before you move a piece that sits on a long line, glance behind it. If a slider would then hit a king or queen, you are looking at a discovery.