Whent he Devas and Asuras struggled together, both of the race of Pragâpati, the Devas took the udgîtha (Om), thinking they would vanquish the Asuras with it.

In plain words: A story begins. The Devas (gods) and the Asuras (demons), both children of Prajapati the creator, were fighting. The gods took up the chant of Om, hoping it would help them defeat the demons.