This is the end of the Gospel of John. Here the author, the Apostle John, gives a closing comment about himself and what he has written in this book. (See: [[rc://en/ta/man/jit/writing-endofstory]])
Here "testifies" means that he personally sees something. Alternate translation: "who has seen all these things" (See: [[rc://en/ta/man/jit/figs-explicit]])