You must have come across with the different status for your sent messages and might have been confused on what does they mean exactly. Below are some of the common status with description.
Delivered/DELIVERED/DELIVRD : NORMAL DELIVERED
EXPIRED/UNDELIV/FAILED : UNIDENTIFIED_SUBSCRIBER & ABSENT_SUBSCRIBER_SM
*A message also gets expired if the maximum retry times out.
Rejected : Blacklisted Sender ID or Blacklist Mobile Number
Above are the common reason for failure that you will see. In case of any doubt related to failure, feel free to open a ticket for the same.
You can also check our FAQ on Reason of Failures for better understanding. You will find it here.