Who is “The Prophet” of whom John writes in this verse and in verse 25?
It is a reference to Deuteronomy 18:15
A prophet like me will the LORD, your God, raise up for you from among your own kindred; that is the one to whom you shall listen.
and also Deuteronomy 18:18
I will raise up for them a prophet like you from among their kindred, and will put my words into the mouth of the prophet; the prophet shall tell them all that I command.
This is why later on in the first chapter of John it is said:
Philip found Nathanael and told him, “We have found the one about whom Moses wrote in the law, and also the prophets, Jesus, son of Joseph, from Nazareth.”
The promise of a prophet was understood to refer to any future prophets but also to a great prophet who would be a transformative figure like Moses.