From the introduction:
Over ten million years ago, an advanced civilization known as the Precursors controlled the galaxy. Having reigned for billions of years and reached the pinnacle of technological advancement, they seeded the galaxy with life and created many species, including humans and Forerunners. The Precursors held the Mantle of Responsibility, the guardianship of all life in the galaxy. They eventually decided to pass the Mantle on to one of their creations; humanity was deemed most suitable for this task. The Forerunners, enraged by their creators' favoring of humanity over themselves, rebelled against the Precursors and wiped them out almost entirely. Some Precursors survived by reducing themselves to dust that, although meant to regenerate their original forms in time, was corrupted over the passage of millions of years and would eventually manifest itself in a completely new form - the parasitic Flood.