If you have not used a rooted Galaxy S5 before, you probably want to read the rest of this tutorial on how to backup and restore ROM on your S5.
What is a ROM backup?
A ROM backup consists of everything on your phone, your whole OS along with all of your settings, apps, app data, and everything else except the contents of your personal files such as videos, photos, and others. Backing up a ROM means you are making an exact 100% copy of your current OS. Whether you are running stock ROM or CyanogenMod11, backing up ROM will backup your current ROM. Think of it as System Backup for Windows or TimeMachine for OSX.