What does "Short Corridor" mean in Varifocal Lenses ?

Typically modern varifocal lenses requie a minimum of 21mm from the point on the lens which is centred on the eye (Iris) to the bottom of the lens in order to provide a comfortable transition space for near vision.

This means that the frame needs to be relatively large.

"Short Corridor" lenses are specially manufactured to allow for a shorter distance (as little as 13mm) between those points - this allows varifocal lenses to be fitted to much smaller (less lens depth) frames.