And it was in the fifteenth year of the rule of Tiberius Caesar

(Bible quotes are from the New Revised Standard Version, unless otherwise noted)

My own translation from the Greek:

And it was in the fifteenth year of the rule of Tiberius Caesar, Pontius Pilate being ruler of the Jews, and Herod being Tetrarch of the Galileans, and his brother Philip being Tetrarch of the Iturians and the Trachonitis region, and Lysanias being Tetrarch of the Abilenes, with Annas and Caiaphas high priests, the word of God came upon John, son of Zacharias in the desert.

And he came into all the surrounding regions of Judaea proclaiming baptism of repentance for forgiveness of sins, as it has been written in the book of the words of Isaiah the prophet,

‘A voice shouting out in the desert,

   Prepare the way of the Lord,

      Make his paths straight.

Every valley will be filled

      and every way and hill will be made humble,

and the crooked roads into straight

     and the rough into level roads;

and all flesh will see the salvation of God.'

1, 2 In the fifteenth year of the reign of Emperor Tiberius, when Pontius Pilate was governor of Judea, and Herod was ruler of Galilee, and his brother Philip ruler of the region of Ituraea and Trachonitis, and Lysanias ruler of Abilene, during the high priesthood of Annas and Caiaphas, the word of God came to John son of Zechariah in the wilderness.

Luke was careful to place John's appearance in historical perspective, just as many of the prophets were placed in the Jewish Scriptures. In this way he lets us know that John was a prophet.

3. He went into all the region around the Jordan, proclaiming a baptism of repentance for the forgiveness of sins,

NLT: Then John went from place to place on both sides of the River Jordan, preaching that people should be baptised to show that they had turned from their sins and turned to God to be forgiven.

John travelled and preached all over the areas that Jesus later covered in his ministry. His message was for all people.

4.: as it is written in the book of the words of the prophet Isaiah,; "The voice of one crying out in the wilderness:; "Prepare the way of the Lord,; make his paths straight.
Every valley shall be filled,
and every mountain and hill shall be made low,
and the crooked shall be made straight,
and the rough ways made smooth;: and all flesh shall see the salvation of God.' "; NEB:; 'A voice crying aloud in the wilderness,; "Prepare a way for the Lord;; clear a straight path for him.
Every ravine shall be filled in,: and every mountain and hill leveled;
the corners shall be straightened,: and the rugged ways made smooth;
and all mankind shall see God's deliverance." '

The quote that Matthew used here from Isaiah 40 gives the impression that John's task was to make Jesus' job easy. We know that Jesus' task was far from easy and that he faced opposition from many quarters. John's message obviously raised people's interest and maybe many of those who flocked to hear John and to be baptised by him also turned up to see and hear Jesus when he began his ministry.

The quote from Isaiah 40 seems to describe a world with all its imperfections put right. John came to prepare people for that possibility. The quote reminds me of the passage Jesus read before his sermon in the synagogue at Nazareth [Luke 4:18, 19] which was also from Isaiah [Isaiah 61:1, 2] and talked about bringing good news to the poor, freedom for captives, recovery of sight for the blind and the coming of the 'year of the Lord' - or the reign of God. The passage Jesus quoted also speaks about putting right things that are wrong in the world. That was Jesus' mission and must be that of his followers.