Relative pronouns: These pronouns are used to answer clauses such as who, which, whose, that, where, and when.
Example: Anna's brother, who loves pickles, made fried pickles for his dinner party.
Who loves pickles? Anna's brother. That is the relative pronoun.
Interrogative pronouns: These pronouns answer a question, but do not answer a clause. These sentences are always in a question form.
Example: What time is the festival tonight?
Hope this helps!