What is "Background App Refresh" on iPhone?

Background App Refresh:

After you switch to a different app, some apps run for a short period of time before they're set to a suspended state. Apps that are in a suspended state aren’t actively in use, open, or taking up system resources. With Background App Refresh, suspended apps can check for updates and new content. 

If you want suspended apps to check for new content, go to Settings > General > Background App Refresh and turn on Background App Refresh. If you quit an app from the app switcher, it might not be able to run or check for new content before you open it again.

Switch apps on your iPhone, iPad, or iPod touch - Apple Support

Contrary to what the previous poster said, leaving Background App Refresh on actually uses more battery. If you turn it off, your apps will not attempt to check for new content while suspended. I recommend switching it off for most apps. Some apps may be slightly slower to launch and update. I've never noticed much of a problem

Leaving it on will also use more data.

1. It is meant to allow apps to refresh their information in the background so it's up to date when you go back to the App. Like fetching email, or updating scores for sports events in sports App etc...
2. Each App in Settings will have its own switch for App Refresh if it requires it and should be possible to turn it on or off there.
3. If you switch it off, the Apps will not refresh in the background, and when you go to use one, you may need to wait for it to refresh to update data.
4. If it's on, it may use cellular data to refresh its information. Certain Apps may use more than others. For instance email apps like Gmail may try to fetch email from the google servers more frequently if Background Refresh is on and fetching email will use data. As mentioned you can set it to Wi-fi only, so apps only try to refresh in the background when connected to WiFi.

If turned off, the iPhone will use less battery as apps are not constantly connecting to their servers for up to date information.

If the switch is disabled, go into Settings->Screen Time->Content & Privacy->Background App Activities, and make sure its set to Allow.

Backround app refresh maximises your battery you can change if you want to refresh only on wifi or turn it fully of

To make the apps refresh on wifi only then:

1.open settings

2.tap”general”

3.tap”background app refresh ”

Then you can change background app refresh to wifi only