We got to learn a bit more on the backstory of the priests asking John. John was doing some baptizing (going underwater and back out). The priests were sent by the Pharisees, who would later on have a bunch of issues with Jesus. And what they are asking about is John's baptism.
So, why was John baptizing in the first place if John isn't Christ or Elijah or even a major Prophet? Most likely that John was preparing himself (and the people who he baptized) for Jesus's coming. Baptism is often known as a symbol of renewal. As stated in an earlier passage, John knew that the One (later known as Jesus) would do so much, so John knew that the One "is preferred before [him]" (1:27).
As a side note, John isn't willing to lose the sandal strap of the One. That definitely shows the dedication John has towards the One. One day later, John will get to see in person the "One".