That is (ουτος εστιν). The one just mentioned in verse John 21:20, "the disciple whom Jesus loved."

And wrote these things (κα ο γραψας ταυτα). Here there is a definite statement that the Beloved Disciple wrote this book.

We know (οιδαμεν). The plural here seems intentional as the identification and endorsement of a group of disciples who know the author and wish to vouch for his identity and for the truthfulness of his witness. Probably we see here a verse added by a group of elders in Ephesus where John had long laboured.