There are lots of examples, diagrams and explanations for all levels of reader ray tracing experience, and even a video to demonstrate how ray tracing could affect game play. Be sure to check it out!
Artistic representation of a building with 4 levels with each 4 rooms
When ray tracing, we want to figure out whether a piece of geometry is hit or not. In ray tracing speak we talk about a “camera” and when we shoot rays through it to determine what is visible we refer to these as “eye rays” or “primary rays”. Naively, we could just shoot eye rays everywhere and see what gets hit and what does not. Clearly, in the above example 15/16 of these rays would be wasted. Thinking about this situation a little, it seems obvious that it is very unlikely that the camera will “see” anything in the right-most room on the lowest-level, so why even bother checking if a ray hits any of the geometry from that area?