Each range of contacts allocated to your plan enables you to send a certain number of Messages. Messages are the total number of SMS, Facebook Messenger, and Emails sent throughout your billing period.
That cap is 'fair use'. Messages cap will suspend the delivery of any upcoming messages until the next billing cycle. This means that if your billing period started on the 1st of November and you have reached your messages cap on the 20th of November, you will not be able to send any communications until your next billing cycle begins on the 1st of December.
You can check your allowance by going into your account: