I'll try explaining via an example for better understanding:
Assume a scenario where you have to order something for the office, say copier paper. In SAP MM, you would create a purchase requisition. This would then per some workflow either get created into a PO directly or after the approval of a manager or more. Once the PO is created, it is sent to the vendor. Now you wait, till the vendor sends you the goods and they are received, at which point you do a goods receipt in MM.
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:
Now, assume using SRM for this same function. In SRM, your supplier/vendor is more tied to the situation. So for example, when you want to buy the paper, you will directly see the vendor's catalog, not your own material master. And the order will be programmed to go directly to the vendor's system, and you can track the status much more easily. So while MM talks to the vendor via documents (PO, Invoice etc), SRM is much better integrated with the vendor's system, so you can be a lot more efficient in your interactions. The vendor will be able to give you a better service.