Handlebar diameter. Set this to the desired handlebar diameter. Some stems come with a shim that makes them fit either size of handlebar. These will be listed with both numbers, separated by a slash. Eg: 25.4/28.5 means the handlebar clamp is 28.5, but has a shim so it will work with the old 25.4 bars. Older mountain bikes like Yellow Brodie 1990's have a bar diameter of 25.4. So if you were searching for stems that would fit those bars, you'd filter on 25.4, and you'd see both ones with and without the shim.
This Bar Diameter filter is a string search, so if you search for "25.4" it will match a string like "25.4/31.8".
The inch equivalent of 25.4 is 1 inch, and 31.8mm is 1-1/4"
From the document: Bike Stem - Clampon Help