I'll try to make an even simpler explanation. :-) Most games use scanline rendering. The program starts with a list of all the objects in the scene, and one by

I'll try to make an even simpler explanation. :-) Most games use scanline rendering. The program starts with a list of all the objects in the scene, and one by one calculates where they go on the screen. Raytracing starts with all the pixels on the screen, and one by one calculates what goes in each pixel. Adding shadows with scanline rendering is hard. Checking every object and calculating where its shadow goes is complicated. (Look up volumetric shadows and shadow maps if you want to know more.) Raytracing shadows is easy. For each pixel, you check if anything is blocking the light. If yes, it's in shadow. If no, it's lit. Adding reflections with scanline rendering is almost impossible (ignoring hacks that only work in some situations). Since one object can have many reflections, you can't just go down the list of objects and calculate a reflection for each one. Raytracing reflections is easy. For each pixel on a reflective object, you check to see what would cast a reflection there. Now, nobody is actually building fully raytrace games. "Raytraced" games first draw everything with scanline rendering, then go back and use raytracing to add shadows and reflections. I still feel like that was more like an eli10 than an eli5, but good enough. :-)