If you use an interchangeable-lens camera, your camera probably has a weird symbol that looks like a circle with a line through it. If you’ve never learned what this symbol is, here’s a 2-minute video by ZY Productions that explains the film/sensor plane indicator.
The symbol is a visual reference for where the plane of the sensor (or film) in your camera is, allowing you to measure focus. Focusing distance is measured from the surface of the sensor or film, not the front or back of the lens — a common misunderstanding. Thus, to accurately measure focusing distance, you need a marking on the body to tell you exactly where to measure from.
This isn’t as useful for still photographers — it’s much more used in the filmmaking world — but it’s still a good thing to know to have a better grasp of how your camera works.