Global illumination (GI) is the simulation of light that bounces off surfaces and lights other surfaces in turn — as opposed to direct lighting, which only counts light that reaches a surface straight from the source. In real life, light bounces constantly: sun hits a red rug, the rug becomes a secondary source illuminating the underside of a table, the table reflects light back to the wall behind it, and so on through dozens of bounces.
Without GI, renders look flat and overly dark in shadows. With GI, shadows pick up color from nearby surfaces, white walls glow faintly in the color of the floor they sit on, and the room as a whole has the soft warm fill that makes architectural photography look architectural. GI is the most important single feature separating photorealistic rendering from technical-diagram rendering.
Modern AI rendering bakes GI into the model. The neural network has learned what real bounced light looks like from millions of training images, so the output naturally includes the color bleeding and soft fill that GI provides. You don't need to ask for it explicitly, but prompts that mention 'naturalistic indirect light' or 'soft bounced light from the floor' will lean into the effect.
See also