After the big talk with Nicodemus, John went to do some more baptizing (going in and out of water). However, Jesus also managed baptizing, and eventually, John had less people getting baptized by him.
Because of the difference in baptism quantity, there are some thoughts raised from John's disciples and Jews on how Jesus does more baptisms than John. It's likely that they thought of John as "the baptizing guy" and that John used to be popular (especially since some concerns were raised from John's disciples). So when "the baptizing guy" doesn't do as much baptisms, it's probably going to raise some eyebrows.
However, as we'll see later, John is actually okay with doing less baptisms. In the following verses, John would respond to the questions. In summary, John explains that once the real deal is here, John wants Jesus to shine bright. He even brought up some notes on who Jesus is.
Jesus's popularity in baptism is because of God wanting it to happen. John knows that too, so John wants to let Jesus baptize more. (3:27)
John is not the Christ -- Jesus is. (3:28)
John compares himself to a friend of a bridegroom. When the bridegroom is ready, it's the bridegroom's spotlight rather than the friend. John acts similarly. (3:29)
John knows that Jesus is from heaven, so Jesus is above all. In contrast, John is only a guy on Earth. Hence, John wants to decrease himself so he can increase Jesus's significance. (3:30-31)
In other words, once the real deal is here, John wants the Lord and Savior to shine.