That marks the location of the sensor (or the film plane on a film camera). You won't often have any commerce with it, but it is the "start point" when talking about focus distance. If a lens says on its spec sheet that its closest focus point is, say, 45cm, it means 45cm from that plane. (Because of the viewfinder and prism housing, it's not practical to mark on the camera body where the sensor is actually located).

It's used mostly for macro photography, where the difference between "rough distance" (the distance from the camera to the subject) and actual distance makes a difference when calculating exposure or magnification.

The exact distance between the subject and the film plane/sensor is used to calculate the reproduction ratio (the relative size of the subject on the sensor) and exposure compensation. The aperture you set on the lens is relative to the focal length of the lens, which is the length of the light path when the lens is focused at infinity. At macro distances for most lenses, the effective length of the lens is longer, so the effective aperture is smaller. If you are not metering through the lens, you need to compensate.