What does John 21:24 mean?

This verse acts like an endorsement, or a notary's seal, on the gospel of John. John is infamously modest in his writing, despite his eyewitness testimony driving the entire content of the book. Most likely, "the disciple" mentioned here is John, confirming he is anonymous person mentioned in other passages (John 1:37; 13:23; 18:15–16; 19:26; 21:23). The last words of this text, coming after John identifies himself, are the only ones in the gospel of John spoken directly from an explicitly first-person perspective (John 21:25).

In the ancient world, it was common for letters to be dictated, with a scribe—called an amanuensis—writing the words as they were spoken. Paul, who may have suffered from poor eyesight, seems to have used this method often (Romans 16:22). At times, he may have hand-written the "signature" of his letters (1 Corinthians 16:21; Galatians 6:11). Scholars speculate John may have done the same with this gospel. That would make the first part of this sentence his official claim to be the author.

The final phrase in this verse, however, is clearly not John speaking. Most likely, this was another note, made by the amanuensis or by the church with whom John was serving at the time.