According to John, this verse means that he is sharing the true story of what God said and what Jesus showed him, like writing in a diary about all the amazing things he witnessed.
Revelation 1:2: Who bare record of the word of God, and of the testimony of Jesus Christ, and of all things that he saw.
This verse states, "If I bear witness of myself, my witness is not true," which contradicts Revelation 1:2 as it emphasizes the testimony of Jesus Christ shared by John.
John 5:31: If I bear witness of myself, my witness is not true.