This means there was a problem when your phone tried to update the app. Several things can cause this, such as you phone ran out of memory or had a bad internet connection.

Most often it's caused because your phone was connected to the aircraft wifi when and tried to update the app. Of course the aircraft wifi doesn't have any through put, but your phone doesn't know that. So it starts the update process but can't complete it. After numerous attempts your phone gives up and any apps that were affected just say "Waiting".

Hotel wifi that requires a sign in can cause the same issue.

To fix it, connect to a "real" wifi source, such as your house. Then restart your phone.

If you don't have access to a real wifi source, you can go to the App Store, tap the circle with the square in it to stop the update process.  That will change in to an Update button that you can tap to restart the process.  

One of those two methods will usually work.