To be on one's side is an idiomatic expression that indicates that that person approves of and will help another. Alternate translation: "Yahweh approves of me and will help me" (See: [[rc://en/ta/man/translate/figs-idiom]])
It is implied that Yahweh is the one who will defeat the writer's enemies, while the writer looks on. The full meaning of this can be made clear. Alternate translation: "I will see Yahweh defeating all those who hate me" (See: [[rc://en/ta/man/translate/figs-explicit]])