In descriptive notation, the square King Bishop 2, which is the square in front of the kingside bishop taken from the player's individual perspective. Therefore, in algebraic notation it is the square f2 or f7 for White or Black respectively.
KB2 is a particularly weak point in the opening, specifically before a player has castled, as it is the only square on a player's second rank that is initially protected only by the king. As a result, it is frequently the target of early game combinations and oftentimes even sacrifices are made profitably to exploit the square's weakness. The Legal sacrifice and Fianchetto are particularly well-known cases of such combinations.