I don't think they've ever referenced something before precursors. The precursors' technology and power is hard to grasp. The forerunner saga books touch on it a little bit. They were basically gods over all life in the universe. They decided what species were worthy of evolution and what species needed more time. The wildest specific thing I can remember is that they had bridges that connected all the planets in a galaxy, like a literal space highway. I think the bridges were composed of hard light.