The Precursors are an ancient alien race from the Halo Universe. They are said to have traveled millions of dimensions, seeding them with life. Much of their technology is based on neural physics, a precursor philosophy that said the entire universe was living, but in a way that was beyond the comprehension of biological organisms. This made their structures nearly indestructible. They are said to be trans-sentient beings, having the ability to travel among galaxies and accelerate the evolution of intelligent life.
When they arrived in the Milky Way to create life again, they chose humanity for the Mantle, not the Forerunner race. The Forerunners were angry about this and tried to kill the Precursors. The Precursors were driven out of the Milky Way, but swore to take revenge. They created The Flood to kill all life in existence.
It is also theorised that the The Flood are not just the tainted remains of the Precursors but the actual Precursors themselves, suggesting that their reasons for seeding and accelerating life across the Halo Universe is not for benevolent reasons but rather as a food source or to satisfy the Living Universe so that it can feed off the emotions and feelings of all life. Such a theory however should be taken with a grain of salt.
The Precursors are highly advanced and are the only Tier 0 race on the Halo Tier scale or at least a 11.1 civilization on the Kardashev scale.