In the opening, it is crucial to keep king safety in mind. Weakening the position of the king can lead to quick losses, or force the sacrifice of material to keep our king from being checkmated. Similarly, if the opponent's king looks vulnerable, it is important to exploit this before the king can find a more secure position.
Often, the f-pawn (f2 for White, f7 for Black) is the weakest point in the opening for each side. The diagram above arises after the moves 1. e4 e5 2. Nf3 f6? 3. Nxe5 fxe5 4. Qh5+. White is taking advantage of the weak e8-h5 diagonal created by Black's second move, and has a large advantage.
Sometimes, these weaknesses can even result in quick checkmates. One example which works on the same idea of weakness along the king's diagonal is the Fool's Mate.
