Global illumination enhances the realism in rendered images by simulating all light interreflection effects in a scene (except caustics). It generates such effects as "color bleeding," where for example, a white shirt next to a red wall appears to have a slight red tint.
The mental ray renderer offers two distinct toolsets for achieving global illumination: photon tracing and final gathering. The primary difference between the two is that photon tracing goes from the light source toward the ultimate illuminated target (taking bounces into account), and final gathering goes the opposite way: from the illuminated surface toward the light source. You can use either of these toolsets separately, or combine them for optimal rendered results.
Scene rendered without global illumination
Same scene with global illumination
Global illumination made smoother by final gather
To calculate global illumination, the mental ray renderer uses the photon map technique.
Using a photon map can cause rendering artifacts such as dark corners and low-frequency variations in the lighting. You can reduce or eliminate these artifacts by turning on final gathering, which increases the number of rays used to calculate global illumination.
You enable global illumination on the Render Setup dialog 
Indirect Illumination panel Caustics And Global Illumination rollout, and final gathering on the Final Gather rollout. In addition, you must designate:
The settings for generating and receiving global illumination are on the Object Properties dialog mental ray Panel. By default, all objects in 3ds Max are set to generate and receive global illumination.