M2 SSD is the smaller and lighter version of the mSATA drive, usually you can find them in laptops (Ultra books specifically) because a lot has to be achieved in smaller, compact space.
Its basically a NAND storage (Similar to flash drives) which provide superior speeds compared to the traditional HDD's. You can consider them as smaller brother's of SSD's. (because of their size). Depending on your budget you can go for a Samsung series (these guys are doing a great job is SSD's etc.,) for 6k to 8k you can get yourself a 128 GB and above M2SSD.
You can use this to install your OS, which will significantly increase your boot speed, app speeds, read/write speeds of your OS. You can then use your traditional HDD of your laptop for media/file storage. This way you can be rest assured that your storage is safe in case the OS crashes.