Like this.
Raytracing, as a process, basically involves taking each pixel in a scene, bouncing it off every object in the scene and tracing its route back to the light sources in the scene. It gets vastly more computationally intensive each time you add another light, reflective surface, or especially refractive surface (eg water, glass). It also gets very computationally intense when dealing with ambient occlusion shadows and the edges of shadows.
But the upshot is... well, essentially, photorealism. It's heavily used in prerendered computer graphics (eg movies), but rarely used in videogames because... well... it takes a *lot* of time to render frames. Some "realtime preview" functions in rendering suites handle that by drastically reducing the resolution of areas that change while redrawing, but things like changing camera angles will basically force a full-frame redraw.