The term SAP MDM stands for Master Data Management and MM is the Material Management Module of SAP. Here, SAP MDM is a standalone system of SAP which is integrated with other systems primarily ECC. MDM helps in updating/creation data from a single source to various systems. In other words, you can say that it eases out Data related issues in HUGE organizations.
And, SAP MM is just another module of SAP which may be or may not be working with MDM as add on. Even if you have a fair understanding of MM, it will help an MDM guy but if you don't have any MDM knowledge then it won't create any significant trouble for MM person.
If you want to learn SAP MM from scratch, then here's the right video for you:
If you are more into reading, then do check out this material provided by Intellipaat: