Outpost
An outpost is a square, usually in the opponent’s half, that a piece can occupy without being driven off by a pawn.
Knights love outposts. A knight on d5 or f5, supported by a pawn and safe from …c6 or …e6, is worth as much as a minor piece plus a headache.
White’s knight sits on d5. Black has no c-pawn or e-pawn that can kick it. Trading a bishop for that knight is often Black’s only way to remove it, which is already a small success for White.
If you can force the opponent to create a hole — for example by making them play …e5 or …c5 and leave d5 empty — you have invented an outpost.