The Magnificat is Mary's response in Luke to Elizabeth's greeting, and it is the longest speech given to any woman in the New Testament.
Its content is a great deal sharper than its usual musical settings suggest. God has scattered the proud in the imagination of their hearts, put down the mighty from their seats, exalted the humble and meek, filled the hungry with good things, and sent the rich away empty. The verbs are in a past tense, which most translations render as accomplished fact rather than as hope, the reversal is described as already done.
The model is Hannah's song in First Samuel, sung after the birth of Samuel, and the parallels are close enough that literary dependence is generally assumed: Hannah too speaks of the bows of the mighty broken, the hungry ceasing to hunger, and the raising of the poor from the dust. Luke's canticle is built on it.
The political reading is not a modern imposition. The text has been treated as dangerous by authorities more than once in the twentieth century, and it is a standard reference point in Latin American liberation theology. Whether Luke intended a social programme or a statement about God's characteristic way of acting is a real interpretive question, and the two are not exclusive.
It is sung at Vespers in the Western church and at the equivalent office in the East, which means it is among the most frequently performed religious texts in the world.