@PeterMores that measurement refers to the ski brake width. The same model binding will be available with a few different brakes to fit a variety of ski widths. You want a brake to be just slightly larger than the ski width underfoot.
For instance, if I wanted to buy a set of bindings for the Volkl M6 ski (https://www.rei.com/product/196983/volkl-m6-mantra-skis-mens-20212022) the waist width is listed as 96 millimeters. Say I want to pair them with the marker griffon bindings (https://www.rei.com/product/181058/marker-griffon-13-id-ski-bindings-20202021) which come with brakes to fit 90mm, 100mm, or 110mm. I would purchase the 100mm bindings because the 90mm brakes wouldn't fit around the ski and the 110mm brakes would hang so far over the sides that while skiing the brakes might hit one another.
If your wife already has some skis, the measurements might be printed on the ski, or you can measure them around where the brakes would fit to identify what size brake/binding you need to purchase. If she has a 95mm waisted ski you can always get a 95mm brake and the shop that mounts them will bend the brake arms just slightly to get them to fit.
Hope this helps,