The phrase "set over" is an idiom that means to cause a person to rule someone else. Alternate translation: "I will assign one shepherd to be in charge of my sheep and the goats" (See: [[rc://en/ta/man/translate/figs-idiom]])
Here "David" refers to a descendant of David. Alternate translation: "a descendant of my servant David" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The descendant of David who will be king over the people of Israel is spoken of as if he would be their shepherd. (See: [[rc://en/ta/man/translate/figs-metaphor]])