a rock: a massive block of stone, a cliff or boulder, and from there a name for God as the unmovable place of safety
At its base this word names a solid mass of stone, the kind you cannot move or break: a cliff face, a boulder, the bedrock under the ground. It is a heavier, more monumental word than the ordinary word for a stone you could pick up.
Because such rock does not shift, crack under pressure, or wash away, the word became one of the Old Testament's favorite names for God, above all in the Psalms and in the Song of Moses. To call God a rock is to say He is the high, unbreakable place where a person takes shelter, the one footing that holds when everything else gives way. The same image could be turned against false gods, who are mocked as a powerless rock that cannot save.
The literal sense and the picture of God are not two different words but one. The metaphor only works because everyone already knew what a real rock was: immovable, lasting, a refuge in the wild. A few places use the word inside the name of a particular crag, the way English keeps the plain meaning of a word inside a place name.