The knight has two special properties. First of all, it is the only piece in chess that can jump over other pieces. The black knight on a1 can jump over two rooks: its own (black) rook on a2 and the enemy (white) rook on b2. The other special thing about the knight is that it always ends up on a square of a different color when you move with it.
Look: the black knight is now on a1, a black square. When you make a move with it, it ends up on b3 or c2, both of which are white squares. Something similar applies to the white knight on f5, a white square: it can only jump to black squares.
that a knight that is moved with ends up on white and black squares alternately