a place or source of safety and strength: a fortress to shelter behind, or the protection a person leans on for security
At its core this word names whatever a person relies on to be safe and strong. Often it is a literal fortified place: a fortress, a walled stronghold, or a fortified city that an army holds, attacks, or runs to for cover.
Far more often, though, the writers use it for the protection itself, and above all for God. He is called the stronghold of a person's life, a refuge in the day of trouble, a place of safety for the poor and the needy. The same word can also expose misplaced trust, as when the prophets condemn leaning on the protection of Egypt, or a wealthy man who makes his riches his security instead of God.
What ties these uses together is the idea of strength. This is not merely a hiding place but a place of strength, somewhere firm enough that danger cannot reach a person there. That is why one English word rarely fits: translators reach for fortress, stronghold, refuge, protection, or strength depending on whether the safety in view is a wall of stone or the God a person trusts.