Light in real life bounces around from one object to the next. In a properly lit and rendered scene, we would be able to see sublet bounces of color from one object casting onto another. Some renderers have the ability to calculate these light bounces for realistic results, but the Eevee renderer is a real-time renderer and it needs a little help to show bouncing light. Right now, light might pour through the corners of the room as if the walls weren't there. Reflections on shiny objects seem to show the environment map, even if a red chair is right next to them.
To fix these issues and give Eevee the help it needs, we can use irradiance probes. Irradiance probes store the lighting information with baked-in data. This means that the bouncing light can be calculated ahead of time and the models can access that information in real time and display realistic...