This gives you the option of either allowing the tracking to continue in the background, or to only allow tracking when you are using the app.
This is a very handy privacy feature, and can float to the surface apps that you may have forgotten about, but that haven't forgotten about you!
The only drawback of popups like this if that you have to deal with it there and then, and there's no option to ask it to remind you again, which would be a nice feature.
Another new, and eye-opening privacy feature in iOS 13 is how the operating system informs you about apps that use Bluetooth for a variety of things that are separate to audio streaming to headphones or speakers. In particular, Bluetooth is being used to by companies to track users via Bluetooth beacons.
It's worth noting that if you deny an app from having Bluetooth access, this will not prevent audio streaming, as this is a separate feature.
Build your own IT pro toolbox - Holiday 2019 edition