What stands for the thing(s) that/which: I didn't understand what he was saying. ( the things that he was saying) Is this what you want? ( the thing that you want) What is different from the other relative pronouns in that it does not refer back to a noun: I wrote my essay on a photo which was taken by Robert Capa. ( which refers to a photo ) That's not what we agreed on. ( what does not refer to a noun) For this reason, what can't be used instead of who, whom, whose, which or that in relative clauses: I wrote my essay on a photo what which was taken by Robert Capa. When in doubt, replace what with t he thing(s) that/which and see if the sentence is still grammatically correct.