令人信服 is a fixed phrase meaning 'convincing' or 'compelling'. It is built from 令 (to cause), 人 (person), and 信服 (to be convinced), literally 'to make a person convinced'. You can use it as a predicate (他的解释令人信服) or as a modifier (一个令人信服的证据). It describes arguments, evidence, explanations, or performances that are so strong they make you believe or accept them.