Remember in earlier passages where John talks about testifying Jesus (AKA "showing who Jesus was after the dove thing")? Now John actually does that. When John met with two of his disciples, John told the two about Jesus -- the Lamb of God (1:36). And that's how they started to follow Jesus (1:37).
Eventually, Jesus noticed that the two disciples wanted to follow. Not the Instagram follow, but more of "I'll go wherever you go". In fact, when Jesus asked them about why they were following, they instead focused on where Jesus is going (1:38). From what John showed them earlier, Jesus is shaping up to be a very important person.
When the two learned about Jesus from John and sought him, Jesus invited them to "come and see" (1:39). So not only did John share news about Jesus, but Jesus also shared himself (because he would later do a big service for all of us).
So looking back from John 1:29-39, whenever we experience Jesus's actions, we can tell others about it. Neat.