SAP MM signifies to “Material Management ”. It is a part of the Logistics area which helps to manage the procurement activity of an organization from procurement. So, SAP MM is one of the modules of SAP that deals with material management and inventory management.
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:
It deals with the Procurement Process, Account Determination & Valuation of Material, Master Data (Material & Vendor Master) Inventory Management, Material Requirement Planning, Invoice Verification and so on.