"Simple zip" refers to a straightforward and uncomplicated method of compressing or bundling files or folders into a single file, called a zip file. Full definition
Simple zip up design keeps baby snug without complicated wrapping. (ecobabygear.com)
Ultimately, it doesn't matter which bags or cubes you decide on — you could even use simple zip - lock bags — but make sure you do have some kind of luggage organiser. (neverendingvoyage.com)