There are different types of components to apps for Android phones. There are activities which you see and services you can't see. When it comes to services, there are foreground services and background services.
The difference is a foreground service allows the user to stop it manually while a background service is stopped by the system automatically.
Recently, we changed our service to be a foreground service. This allows the app to keep running and stay connected to your GPS. This means Androids will display a message to let users know about apps that are running services. As a result, we've gotten a lot of inquiries asking why we changed the app, but the Lezyne app is running as it always has. The only difference is that the Android alerts you now and gives you the option to kill the app, and if you aren't in need of any background services like notifications, it's fine to kill the app.
Note for Apple users: iPhones do not require the app to be open for notifications. iOS sends notifications on its own - and it might send the same notification multiple times if you haven't cleared the notification from your locked screen.