The Magnificat is Mary’s response to Gabriel’s message. It’s from Luke 1:46-55, a song of praise to God that begins like this:

My soul magnifies the Lord
And my spirit rejoices in God my Savior;
Because He has regarded the lowliness of His handmaid;
For behold, henceforth all generations shall call me blessed;
Because He who is mighty has done great things for me,
and holy is His name;
And His mercy is from generation to generation

It’s been put to music by many excellent folks. Tavener, Tallis, Palestrina, Praetorius, Monteverdi, Vivaldi, Bach, Mozart, Mendelssohn, and about a million other people. Plus whoever wrote the pretty song in the video above.