Indirect illumination is the method used by 3D software to mimic the behavior of light in the real world, mostly referring to the light bounce effect, enabled by default in modo701.
I have provided a sample figure to demonstrate the light bounce effect in the real world. The image is over-saturated, so the effect gets very noticeable. We have an orange in a white environment, lit by the sunlight. There are lit areas and shadowy areas, but take a look at the zone where the ground meets the orange. You will notice an orange tinting in the ground (the same color as the fruit). That is the light coming from the sun, bouncing off the orange skin and hitting the floor, coloring it with the tone captured from the bounce.
That's the effect we will achieve when using indirect illumination, and that's the way to create a radically different look...