A knight is a piece in the game of Chess, which moves in an "L" shape (two squares like a rook, and then one more square at a right angle to the first motion). This means that it will always land on a different color square than the square it was previously on. The knight is also the only piece in chess which can "jump" over other pieces (either friendly or enemy pieces).
An attack over a long diagonal spanning nearly the entire board is both very effective when used on offense, and something to beware on defense.