Comment by SapThePixelFox

You should read scratchapixel really)

https://www.scratchapixel.com/lessons/3d-basic-rendering/int...

In the real world light travels from the light source to the eye. But this lesson xplains that simulating this with a computer is not efficient. A more efficient technique is to follow the path of light but in reverse, starting from the eye and waking back to the lights making up the scene. Again read the lesson(s) it's all explained and that's why the lessons are there in the first place.