My Outbox shows a number. What does that mean?

My Outbox shows a number. What does that mean?

Sometimes the messages scheduled to send may remain unsent due to some errors. In such case, the Outbox will display the error messages with a red warning icon.To send them, open the Outbox, select those messages and click Send Immediately. If the error was a temporary error, the messages will be sent upon 'Resend'.