卫 is a bound morpheme (不自由语素) meaning 'to guard, to defend' that appears as a component in many compound words. You will see it in words like 保卫 (to defend), 卫生 (hygiene, literally 'guard life'), and 卫星 (satellite, literally 'guard star'). As a surname, it is pronounced Wèi. Because 卫 cannot stand alone as a word, always use it within a compound.