MDM stands for Mobile Device Management. An MDM can be used to control and keep track of mobile devices such as cell phones. MDM policies can be applied to company issued devices or if the company allows for BYOD (Bring Your Own Device). With BYOD, the MDM would aim to only control work related items, and leave the personal items alone.
MDM policies can be used to define password policies on mobile devices. It can also disable features such as the use of the camera, app downloads, etc.