Expands to depth of field preview: a feature that used to be present in most SLRs, and that can currently be found only in high-end models.
It means that the camera is able to stop down to its working aperture so that, in the viewfinder, you can have an idea of what the depth of field will be.
Depending on the lens technology, DOF preview can be activated by a mechanical button requiring quite a bit of force, or by an electrical switch.

Of course, this feature can be present only in relatively modern SLRs: the old ones, that employed stop down metering, did not need DOF preview.
It is mostly useful with telephoto lenses and when there is a lot of light: with shorter lenses the effect is not very visible, and in dim light the viewfinder darkening will prevent the photographer from evaluating sharpness (especially with microprism and split image viewfinders)