A relative pronoun is a DMW; i.e. it introduces a dependent clause. Who, whom, which, that, what and whose are the relative pronouns. more A relative pronoun is one of the two kinds of DMWs. When the DMW is a relative pronoun, it indicates that the dependent clause is most likely adjectival.
In Greek, the relative pronoun is declined for number, case, and gender. It is distinguished from the article by reason of the fact that it always has a rough breathing mark which the article usually does not. Study chapter 14 in BBG. Here is the complete paradigm:
The word אֲשֶׁר is the relative pronoun in Hebrew; see page 8 here or this video. It usually follows the noun it modifies and introduces a relative clause. Its translation depends on the noun:
if the noun is a place, it means “where”
if the noun is a person, it means “who” or "whom"
if the noun is an object, it means “that” “what” or “which”
In relative clauses, you will sometimes see a pronoun that refers back to the אֲשֶׁר. video or KHW 22.8a