علم الرجال refers to "Criticism of Hadeeth narrators الجرح والتعديل " studies and evaluates the accreditation of Hadeeths and their narrators.
The science of Al-Jarh wa At-Ta‘deel الجرح والتعديل or 'ilm ar-Rijaal علم الرجال (Criticism of Hadeeth narrators) studies and evaluates the accreditation of Hadeeths and their narrators to determine if they fulfill the conditions which qualify the narration to be authenticated, and to find out the reasons behind any possible rejection of their narrations.
The chain of narrators which fulfill the conditions of authenticity are acceptable; other chains which include one or more reason to justify their rejection means they are rejected.
O you who have believed, if there comes to you a disobedient one with information, investigate, lest you harm a people out of ignorance and become, over what you have done, regretful. - 49:6
The knowledge of al-Jarh wat-Ta’deel has been proven in the Kitab (Qur’an) and Sunnah and by Ijma’ (consensus). So this ayah is a clear text about the acceptance of the news of one trustworthy person. And the point of that is that Allaahu subhanahu wa taa’ala commanded confirming and verifying once some news comes via a faasiq (a disobedient sinner). And the understanding of that is that the trustworthy narrator does not require confirmation of his news. And the trustworthy narrator is the one who is known for truthfulness, trustworthiness and verification in narrating his reports. And this ayah is also a proof for splitting the people up into two categories.