The red alert icon with the number in it probably means you have data to be downloaded and updated. The higher the number, the more alerts there are / stuff there is to be downloaded.

If the icon is over the Planning button, then it indicates that there are messages for you to read, typically relating to an active flight plan.

Make sure you are connected to the internet (preferably WiFi if you need to download a large data set), then tap on the alert icon to see where any messages are. If there is a red update button in Settings>Data Downloads, tap on it and your data will be updated. That may take a few minutes depending pin how much data there its and the quality of your internet connection.

Sometimes the Alert icon is just letting you know that there has been a data issue, and AvPlan is getting on with fixing it in the background. Just tapping on it might make it go away so quickly that you cant even see what the issue was. This a good thing!