Here to "reign before God" means that God will approve of what the king does and bless him for ruling with justice. Some versions translate this as the speaker's desire: "I desire that the king reign before God forever" (See: [[rc://en/ta/man/translate/figs-explicit]])
# appoint your steadfast love and faithfulness to protect him
Here "steadfast love" and "faithfulness" are spoken of as if they were guards that would protect the king. Alternate translation: "always protect him because you love him and are faithful" (See: [[rc://en/ta/man/translate/figs-metaphor]])