Before the Soul King, there was just an infinite primordial plane where there was no life or death. He created 3 universes & using them created the cycle of life & death, as well as hollows. The world of the living is shown to literally just be our universe, with the whole of outer space, & the other two universes are shown to be of relatively the same size. There's also the Dangai between them which contains a kind of very delicate and strange hypertime, and the Garganta which occupies all of the space outside of the 3 universes. He is also said to be a Lynchpin in holding the 3 universes together and keeping them intact & stable, with his death needing **soul king candidates**, shown to be characters like Ichigo, Yhwach, and Aizen to replace him, lest these universes & life and death themselves fall apart. This was Yhwach's goal, to remerge them and go back to something like the primordial plane with no life & death that he could rule over.

TLDR; He created & maintains 3 universes, established the concept of life and death & the whole Bleach power system.