Momodot, Radiosity is a name for a natural phenomenum: light is reflected by materials, I mean objects with certain physical properties. For example, a mirror reflects the light very much but the reflected rays are not disturbed, obeying to the classical optic laws... But a wooden ground on the contrary will difuse the reflected rays in all directions because its surface is not a perfect plane, it has many many little bumps... Those reflected rays will then lighten the objects around which will once again difuse the light and so on. After each reflection, there is is loss of intensity as a part of the energy the light carries is absorbed by the hit objects. This produces very soft shadows and an ambiant lighting that is very hard to get with common lighting system. See the attached image... :)