The Global illumination section adds the light that arrives after hitting something. Without it, whatever gets no direct light is black. With it, surfaces light each other up: a red wall tints the floor red, and objects that emit light brighten what is around them.

Global illumination starts off.

Controls

Enable global illumination: Turns indirect light on.
GI samples: From 1 to 1024. More samples remove grain and take longer.
Diffuse bounces: From 1 to 8. One bounce already lifts the black out of shadows; closed rooms keep getting brighter up to the third.
Intensity: From 0 to 4. The strength of the indirect light.
Firefly clamp (0 = off): Removes isolated very bright dots, darkening the indirect light a little. At 0 it is off.
Irradiance cache: Speeds up the image a lot, especially on smooth surfaces. On by default.
Cache spacing (pixels): From 1 to 16. Higher values are faster and smoother, and can lose small shadow details.
Noise reduction: Removes the grain of the indirect light at the end of the image, without blurring textures. On by default.

Limits

Indirect light does not go through glass or reflections, and the water shader does not take part in it. Very tight corners may come out a little brighter than they should.