'Message Sent and Not Received' means those text messages sent from Campaign Builder that have NOT successfully been delivered to your Contact.
There are numerous reasons for a text message not being received by your contact.
eg. A wrong mobile phone number, a wrongly selected country code or a technical delivery issue within the telecommunications company in the delivering country sent, to name but a very few.
The total number of 'Messages Sent and Not Received' from the start date of your current subscription can be viewed in the 'Message Status' window in the Dashboard.
Greater detail regarding the Time, Date, Phone number, message content and if the message was sent successfully or not can be viewed by going to the top navigation 'Reports' and selecting 'Message Reports.' and downloading the CSV report file.