The phrase "open-aperture metering" contrasts with an older, stopdown metering scheme, used in the earliest SLRs to offer TTL exposure readings. In open-aperture metering, a linkage at the rear of the lens communicates the currently-selected f-stop to the camera's exposure meter, while leaving the lens at its widest aperture for a bright viewfinder image. This lets the meter weigh the amount of dimming which will occur as the diaphragm closes to the selected aperture. (An automatic aperture then stops down in the instant before the shutter opens.) So unlike stopdown metering, there is no need to have the viewfinder inconveniently dim while taking meter readings.
Open-aperture metering has been standard for SLR and DSLR cameras since the mid-1970s. Earlier SLRs or budget models (notably, ones using 42mm screw lenses) may be assumed to lack open-aperture metering, although there are exceptions.