King safety is a measure of how exposed your king is to attack, governed by whether it has castled, kept its pawn shelter intact, and has pieces nearby to defend it.
Almost every attack is ultimately aimed at the enemy king, so its safety underpins the whole game. The three pillars are getting castled early, keeping the pawns in front of the king unmoved or only lightly disturbed, and having defenders — especially a knight on f6 or f3 — close by.
A king is in danger when its pawn shield is cracked open (a missing or advanced f-, g- or h-pawn), when defenders have been traded off, or when it is stuck in the centre because castling was delayed. Each of these gives an attacker open lines and squares to exploit.
Before launching your own play, ask who is safer. If your king is the more exposed one, finishing development and tucking it away usually matters more than any pawn you might grab.
Castle early, then avoid pushing the pawns in front of your king without a concrete reason. An unmoved f-, g- and h-pawn shield plus a defending knight covers most of the work.
Whenever the position is open or about to open. Central files and diagonals give the enemy pieces direct routes to an uncastled king, so finish development and castle before the lines crack open.
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