Data Saver is a feature that was released with Android 7.0 that allows your device to prevent some apps from sending or receiving background data while on the network. This helps reduce unnecessary data usage by limiting how much data the device uses. This is a great feature to turn on when you are away from Wi-Fi and need to conserve data usage. Apps that are in use can use mobile data but will do so less frequently and notifications for certain apps will still come through but can be delayed. There is the option to create a customizable list of apps that can use data without restrictions. If you are experiencing issues with delayed notifications or images not loading on web pages, data saver could be affecting these functions.
To turn this feature on/off start in Settings>Connections>Data Usage>Data Saver. When data is on, there will be a circle and cross icon for Android 7.0. On Android 8.0 there will be a triangle with two arrows that show in the navigation bar to remind you this feature is on.