Hold a position the extra material cannot break.
A fortress is a position where the defender can hold despite a material disadvantage. The pieces and pawns deny the attacker a useful entry or breakthrough. Check every pawn break and sacrifice before deciding the position is secure.
Bring your king toward the enemy king
Your king moves closer to the other king. Your king and pieces need to work together to limit its escape squares.
2 more clean runs (at any point) to reach Recall.