to put your confidence in someone or something, leaning on it for safety; and so to feel secure
At its heart this is the verb of leaning your whole weight on something because you are sure it will hold. It is not a calm opinion that a thing is reliable; it is the act of staking your safety on it, the way a person rests their full weight on a wall they believe will not give way.
Because the word is about leaning, everything depends on what you lean on. When the object is God, it is the steadying confidence the Psalms return to again and again. When the object is anything else, the same posture becomes the great folly the prophets attack: leaning on idols, on silver, on warhorses, on Egypt, on fortified walls, on one's own cleverness. The word itself does not change; the thing trusted decides whether the trust is wise or ruinous.
The verb also reaches past the act of relying to the state it produces: to feel safe, to be at ease, to live without fear. That security can be the deep peace of the one who rests in God, or it can curdle into a false sense of safety, the careless ease of a city or a person who feels untouchable right up to the moment of ruin.