Lots of people are having trouble finding where to restrict background data usage. Most people that are on prepaid plans have a set amount of 4G or LTE allowance before they get very slow speeds from their carriers so it is wise for people with such prepaids plans to restrict background data unless they are connected to a Wifi signal.
Here is how to restrict background data on the Galaxy S5. Tap on settings and then go into Data usage. You will then see 3 dots on the top right corner which when tapped will drop down the following menu which I took a screenshot of.
If you check mark Restrict background data you will see a message popup “Restricting background data will stop some apps and services from working unless device is connected to a Wi-Fi network.” Hit OK and your finished. And if you want to un-restrict it, just do the opposite. This should also work for other touchwhiz devices like the Note 3, Galaxy S3 or S4.
Notice message says “will stop some apps” because it can’t restrict everything. You may have other apps like Tango, Viber etc that can still be using background data. If you uncheck Auto sync, you will stop automatic synching of apps with accounts like Google, Facebook etc.