“David my servant.” This is an antonomasia for the Messiah (see commentary on Ezek. 37:24).
“prince.” The Hebrew word translated “prince” is nasi (#05387 נָשִׂא or spelled נָשִׂיא nasiy). The root meaning refers to being lifted up, and thus it can mean prince, leader, chief, captain, etc. It can also refer to a rising mist or vapor. In this context or in other contexts in which it refers to the Messiah, “prince” is a good translation because God is the king and the Messiah is his only son and rules under Him and with His authority.