What does John 1:21 mean?

In verse 21, John the Baptist's interrogators ask about particular Old Testament prophecies. The Baptist does not claim to be a particular figure, either the return of Elijah (Malachi 4:5), or the prophet which Moses predicted in Deuteronomy 18:15. The only claim the Baptist makes for himself is a fulfillment of Isaiah 40:3, which he states in John 1:23. Notice that the religious leaders are constantly asking questions about their pre-conceived notions. They don't ask the Baptist to explain himself until after they've run out of their own ideas. This is a common trait, even today, in those who are resistant to the gospel. Their first instinct is to criticize, assume, and point fingers. At some point, when all of those poor ideas are exposed, there is nothing left to do but ask what the gospel really means. Even then, as John 1:22 shows, that interest is often just so that they can get the conversation over with.