And John bore witness: “I saw the Spirit descend from heaven like a dove, and it remained on him. (John 1:32, ESV)
Here is John's testimony, his witness. This is the confirmation sent from God that Jesus was really the one that John had been looking forward to. This is most likely referring to the baptism of Jesus as reported in the other gospel accounts. John knows that when he sees this that the one on whom this happens is the messiah.
John the Apostle is sharing this with us in order to make sure that any of John the Baptist's continued followers would know that the testimony (witness) was that Jesus was the Christ.