In Ultra Data Saving Mode, background data will be automatically blocked for all apps by default, but you can allow specific apps to use it.
Data saver helps cut down your data usage by preventing some apps from sending or receiving data in the background.
Apps that you're currently using can use mobile data, but may do less frequently. For example, images may not be shown until after you tap them. You can use your device in a normal way but know that even if you don't close an app down completely, it wont use up any data without your permission.
If you turn on data saver it might mean that some notifications or emails wont come through unless you open up the app and refresh it - but you can easily allow all the apps that are a priority to you full access to data usage.