Zoom range simply means the ratio of minimum and maximum magnification. For many years after variable scopes became popular in the U.S. the most common range was 3X, as in 3-9x scopes.
But many companies eventually developed practical variables with even more magnification range, in part because shooters wanted the maximum amount of flexbility. The most I've heard about is 6X, with examples being 1-6x or 3-18x scopes.
Many companies use the zoom range in their model designations. Swarovski, for instance, has three lines of riflescopes, Z3 (with a 3X magnification range), Z5 (5X) and Z6 (6X).
