An outpost is a square in enemy territory that no enemy pawn can ever attack, usually because the pawns that could have covered it are gone or have already advanced past it. A knight planted on an outpost, protected by your own pawn, is a permanent problem: it can't be chased away, only traded off at a cost.
Knights love outposts more than any other piece because a knight's strength is entirely about its square: on the rim or at home it's mediocre, but anchored on a central outpost deep in enemy lines it can be worth more than a rook. When you spot a hole in your opponent's pawn structure, start maneuvering a knight toward it.
Black has no c-pawn or e-pawn, so no pawn can ever touch d5. White's e4 pawn covers it.
Plant the knight on the square no pawn can ever kick it from.