"Specified Order"? How to specify?

Hey there. I see there’s an option in this plugin to use “Menu/Specified Order” for the sort order, and that sounds like what I’m looking for, but I have no idea how to actually set the order of images. I’m currently using the method of attaching images from the media library to specific pages. Should I be doing something else if I want to specify the order?

Thanks in advance.

https://www.ads-software.com/extend/plugins/really-simple-gallery-widget/