What does the app name means above music player in Control Center Mine is showing "App Store", when clicked, it brings me to App Store, but what is the reason of it showing up in Control center?
The wording used is to represent the source of the audio content you are listening to, but it tries to be clever and give you more than just the app name.  Instead of Safari, it will scroll the URL of the page you are viewing.  Instead of Music, it will pull the artist/track name etc, and instead of App Store, it will display the name of the last app you were looking at (presumably you were watching a preview video for an app in the store)