MDM stands for Mobile Device Management. When a business or educational institution purchases a batch of devices from Apple, they will often enrol them in a program which allows the organisation to add restrictions to all their devices and essentialy have authority over them all. With MDM active on your device, you won't be able to activate it without permission from the supervising authority. Therefore, checking the MDM status has become just as essential as the Find My iPhone status.

Sample:

Case 1: MDM Locked
IMEI Number: ###############
Serial Number: ************
MDM Enrollment Status: ON

Case 2: No MDM Profile
IMEI Number: ###############
Serial Number: ************
MDM Enrollment Status: OFF