IDL (Indirect Lighting) is simply bounced light. If you shine a bright light on a shiny mug, the mug will reflect that light onto another surface, even if the mug itself isn't casting light - like real light does. The mug doesn't even have to be shiny. If it was just a dull white matte it would still reflect light. It's what causes shadows to be somewhat lit rather than deep, dark black blobs :P

Laurie