Black has hung their knight. It is open for white's bishop to capture.
Hanging pieces are classified as pieces that are undefended or "loose." They can potentially be captured, so it's important to search for hanging pieces in your position. Try to defend them or move them to a defended location. See what hanging pieces your opponent has so you can attack them or exploit them with tactics.
To avoid hanging your pieces, always look where you intend to move. Can you be captured? Are you protected there? What threats are you creating and what opposing threats are being revealed by moving such piece? If you spot that your piece will be hanging, then try to find an alternative move or build up for that move.