e.g. Can you repeat what he said?
1. Can you repeat the thing which he said? (what =relative pronoun)
2. Can you repeat___? + What did he say? (what=interrogative pronoun)
(What does the example sentence mean? Does it mean #1 or #2? Is 'what' a relative pronoun or an interrogative pronoun?))
The first sentence and #1 mean about the same thing.
In #1, "which" is a relative pronoun.
In #2, "What" is an interrogative pronoun.