That mark indicates the camera’s focal plane position—the exact plane where the image sensor (or film, on film cameras) sits.

It’s mainly used when you need very precise subject-to-sensor distance measurements. Instead of measuring to the front of the lens or camera body, you measure from your subject to that mark. This matters in situations like macro work, scientific photography, product setup, or any case where exact focusing distance is important.

For everyday photography, most people never need to use it, but it’s a standard reference point found on many cameras.