This is the disciple who testifies to these things and who wrote them down. We know that his testimony is true. Jesus did many other things as well. If every one of them were written down, I suppose that even the whole world would not have room for the books that would be written.
John ends his Gospel by stating that everything he has written is true, but then he says, even so, what has been written is but a small part of a huge story that could never be contained in books, even if those books filled up the whole world. John is saying that Jesus cannot be limited by the usual human limitations and boundaries. The Pharisees and the Sadducees and the powerful religious and political leaders in Jerusalem could not contain him. He could not be contained by King Herod or by Caiaphas or by Pontius Pilate. Even the tomb of Joseph of Arimathea could not contain him. For Jesus has risen and is seated at the right hand of God, and he has given his disciples in Jerusalem, and us, the authority through the power of the Holy Spirit to go and tell the rest of the world the good news that Jesus is the Christ.