Imagine light rays coming out of your eyes. For every ray that hits an object, draw rays to every light source from that hit point.
Light in reverse.
In-between those two steps you can figure out intensity and colour of light, the colour of objects, roughness, transparency, refraction, reflectivity, etc etc. You can even bounce multiple rays to other objects for global illumination and whatever.
Imagine light rays coming out of your eyes. For every ray that hits an object, draw rays to every light source from that hit point.
Light in reverse.
In-between those two steps you can figure out intensity and colour of light, the colour of objects, roughness, transparency, refraction, reflectivity, etc etc. You can even bounce multiple rays to other objects for global illumination and whatever.