When you activate the "Upload in background" option, the myCloud app will ask for permission to access the location of your iPhone/iPad. This allows you to upload photos during an automatic backup even if the app is not open.
This is how it works: usually myCloud can only upload photos when the app is open and for a few minutes after it has been closed. However, if you activate the "Upload in background" option, it is also possible to upload data as soon as soon as your iPhone detects a change in location. Your location will not be evaluated or passed on to third parties as part of this function. Since only significant changes of location are queried, activating this setting does not increase power consumption.
The automatic backup also works when the "Upload in background" function is not activated; however, for the reasons listed above, the app will need to be open regularly for the photos to be uploaded.