The sortkey specifies the order in which images in an album that is sorted by key will appear.

Albums can be sorted either by date or by sortkey, controlled when configuring the album. IF it is sorted key sortkey then the number specified here will control the order in which images appear. Lower numbers appear first.

As a rule of thumb sortkeys are usually specified as 10,20,30 etc but there are no restrictions and multiple images may have the same number. If more then one image has the same sortkey then they will be displayed sorted by name.