"Hidden snaps" refers to fasteners or buttons that are concealed from view. They are typically used in clothing or accessories to secure two pieces of fabric together without being visible on the outside. Full definition
Along the inside of the front / back of the diaper the polyester outer material is used to create a flap that hides the snaps for the snap - in pad holder. (backtocalley.com)
Italian milled fabric Signature curved waistband with hidden snap closure at center front Mesh lining, zip fly, side pockets, and zipper back welt pocket Metal buckle at adjustable side tabs (bonobos.com)
Some examples would be: 1 row of snaps instead of the standard 2, if you are getting covers converted and want me to add pul backing for extra support, If you have an all - in - one that I need to open and add hidden snaps too, If you want me to add a (convertmydiapers.com)