Question: 
What is the The Benedictus?

Answer: 
The Benedictus was the song of thanksgiving uttered by Zechariah on the occasion of the birth of his son, John the Baptist.
The whole canticle naturally falls into two parts. The first is a song of thanksgiving for the realization of the Messianic hopes of the Jewish nation. The deliverance was now at hand, and was pointed to by Zechariah as the fulfillment of God’s oath to Abraham.
Blessed be the Lord God of Israel; because he hath visited and wrought the redemption of His people: And hath raised up an horn of salvation to us, in the house of David his servant: As he spoke by the mouth of his holy prophets, who are from the beginning: Salvation from our enemies, and from the hand of all that hate us: To perform mercy to our fathers, and to remember his holy testament, The oath, which he swore to Abraham our father, that he would grant to us, That being delivered from the hand of our  enemies, we may serve him without fear, In holiness and justice before him, all our days.   Luke 1:68-75
The second part of the canticle is an address by Zechariah to his own son, who was to take so important a part in the scheme of the Redemption; for he was to be a prophet, and to preach the remission of sins before the coming or the Dawn from on high. The prophecy that he was to “go before the face of the Lord to prepare his ways” was of course an allusion to the well-known words of Isaiah 40:3.
And thou, child, shalt be called the prophet of the Highest: for thou shalt go before the face of the Lord to prepare his ways: To give knowledge of salvation to his people, unto the remission of their sins: Through the bowels of the mercy of our God, in which the Orient from on high hath visited us: To enlighten them that sit in darkness, and in the shadow of death: to direct our feet into the way of peace.    Luke 1:76-79
From Wikipedia Benedictus (Song of Zechariah)