What does "Due to inactivity for 15 days, this app has been deactivated." does this mean? How do I reactivate my app?

Since there have been no messaging events (incoming or outgoing) activities in your app for 15 days, the app is considered inactive and is deactivated. Once deactivated, you won't receive any incoming message events for the app.

However, your app can be reactivated instantly. All you need to do is click the reactivate button from the settings page of your App's UI as shown in the below screenshot: