The caption at the bottom changes based on whether you enable/disable.
When disabled it reads
"Disabled for installed apps except for system apps used for Republic calling, SMS and voicemail"
You want this setting when you don’t want any of your apps to give access to cellular data. The apps can still access data when connected to WiFi.
When enabled it reads
"Enabled for all installed apps"
You want this setting when you want to allow all you apps to be able to access cellular data.
If you want to control cellular data access to individual apps you will need to install a third party firewall app such as