Yeah , just like the topic says , What is global illumination and how to use it in BGE 2.79/UPBGE 0.2.5 ? Please explain as much as you can
There are tons of excellent explanations and videos on global illumination if you just Google it.
I like to think of it as “light that comes from everywhere.”
But – as “Midphase” correctly says, you really need to “Google it,” and be very sure that the page you’re reading is discussing recent versions of Blender. Note that different render engines do things (fundamentally …!) differently.