King safety
King safety is how well the king is sheltered from checks and mating nets, usually by castling and a sound pawn shield.
A king in the center is safe while the files are closed. Once the d- or e-file opens, every check becomes a problem. Castling tucks the king behind three pawns and connects the rooks.
White has already castled. Black’s king is still on e8, and the d-file is opening around the pawn on d4. Black can still castle, and should, before the center is fully cleared.
King safety is also about the pawn cover after you castle. Pushing the g- or h-pawn makes luft against back-rank mates, but it also gives the opponent hook squares for an attack.