What does John 1:26 mean?

The Pharisees have challenged John the Baptist's right to baptize (John 1:19–25). Jewish converts were often baptized, but the Baptist was performing this ritual on those who were already Jewish. Even more controversially, he did so without any claim to priesthood or prophet-hood. He is neither the Savior, a re-born Elijah, or the prophet Moses predicted (Deuteronomy 18:18).

Earlier (John 1:25), skeptical leaders essentially asked the Baptist, "Who do you think you are?" He responds that his ministry only uses physical tools, such as baptism with water. The ministry of Jesus will go far beyond physical rituals. The Baptist then immediately declares that there is someone present whom the Pharisees don't know.

He will explain that this "One" is so much greater than him that he isn't worthy to untie his sandals. In this era of history, people walked everywhere and their feet would become covered in grime and animal waste. Tending to someone else's shoes was the lowest chore of the lowest servant. This is how far beneath the Promised One John the Baptist sees himself.