The count of mobile app sessions is the count of launch events in apps. Each mobile app sessions begins with a launch event of the app and ends with 5 minutes (default value) of the application being in background or closed. Launch is calculated client side based on app activity. If an app is opened and closed for 5 times within 15 minutes, it would be counted as 5 Launches.
First Launch is the Launch at the first run after installation or re-installation.