The phrase "be my rock of refuge" is a request for protection. The second phrase emphasizes the first phrase. (See: [[rc://en/ta/man/translate/figs-parallelism]])
Yahweh is spoke of as if he were a huge rock that would protect the writer from attack. Alternate translation: "like a huge rock on which I can be safe" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Yahweh is spoken of as if he were a strong fortress in which the writer would be protected from his enemies. (See: [[rc://en/ta/man/translate/figs-metaphor]])