a man who may go out and come in before them and lead them out and bring them in

This is an idiom that means a person who will lead all the people and also lead the army into battle. (See: rc://en/ta/man/translate/figs-idiom)

so that your community is not like sheep that have no shepherd

This is a simile that means without a leader the people will wander and be helpless. (See: rc://en/ta/man/translate/figs-simile)