John the Baptist is a key figure during the Advent season because he sums up what Advent is: preparing for Jesus, the Messiah.
While John is a somewhat larger than life figure, I don't think we always give him enough thought. He becomes a kind of "given" in the scheme of our Salvation History. He's the guy who baptized the people of his and Jesus' day down by the Jordan River, and yes, he even baptized Jesus, but upon reflection, I find so much more to him.
John knew his vocation and he lived it. In fact he lived it from the time he was in Elizabeth's womb. In the presence of Jesus in Mary's womb, he leaped within Elizabeth. This may have been his first act of evangelization. His little leap led to his mother's recognition of Mary as the "Mother of my Lord" (cf. Luke 1:43) and to Mary's Magnificat. (Luke 1:46~55)
John spends most of his brief adult life living in the wilderness clothed in animal skins and living off of locusts. That, taken out of context, might make John a bit of an eccentric, but as Fr. James Torrens wrote in one of his Advent meditations: "John the Baptist was the last of the great servants preparing God's way. With his rough austerity, he had learned to live wholly off of God." (Daybreaks:Daily Reflections for Advent & Christmas)
I believe that this is what John is telling us as he leads us more deeply into Advent~ that we too should learn to live wholly off of God. We don't need to live in the wilderness clothed as John was or live on a diet of locusts to do it. We can do it in how we live our everyday lives in the vocation God has called each of us to live.
Over two thousand years ago, John was telling the people of that time to prepare the way for God who would walk in their midst in the person of Jesus Christ. The message is the same for us~Jesus is still in our midst in word and sacrament and he will come again in His person at the end of time.
As my pastor said in his homily for this Sunday: our entire life is an advent. If lived according to the message of John and the message of the Gospel, we are preparing the way. We are preparing the way of the Lord in our lives, and in the lives of those who God puts in our path, and that way will lead all of us to our heavenly home.