Here's also some background into the relationship between Forerunners and humans that cause the setting the book is in:

Forerunners and humans had a war known as the great war, in which the Forerunners won but only because humans were having a war on two fronts, one with Forerunners, and one with the Flood, a form of living hunger created by the precursors in an attempt to destroy Forerunners, which humans intercepted, causing them to now be the floods target, destroying their race from within. the humans successfully defeated the flood, but at a cost, because they no longer had the power to defeat the Forerunners. after the war, Humans were de-evolved into several hundred species of life, and quarantined to planets across the galaxy. meanwhile the Forerunners discover the second battle that the humans had fought, fearing the flood now themselves, centuries later, the flood return, and the Forerunners do not know how to control it, so, the Halo's are constructed, which destroy all sentient life across several light years, therefore starving the flood of their food, killing them off if they do not find sustenance in time. on these halo's are 2 of every species so that, life may continue if things do go to hell.