Initial findings of the Canon 6D

It's just a light that comes on if the environment is too dark for the camera/lens to focus.. so by shining some light into the subject the camera/lens would be assisted in its focusing helpful if you try to focus a lot in low light.

Alternatively, by putting an external flash onto the camera, the camera could use the focus-assist light from the flash to assist in focusing, making the AF illuminator a non-issue if you have an external flash attached.