M.2 is the socket. You can have a regular old-fashioned shaped SSD that connects to them or Intel's Optane drives might. For example, this will connect to an M.2 socket via a cable:
https://www.overclockers.co.uk/inte...e-drive-with-m.2-cable-adaptor-hd-08m-in.html
But mostly you're talking about the little stick-style drives like this:
https://www.overclockers.co.uk/wd-g...-solid-state-drive-wds240g2g0b-hd-558-wd.html
You'll almost certainly want to get something like the latter because that's what the motherboard layout will be designed for with the little placement screws and maybe a heatsink. Not that you couldn't connect a different drive via a cable.
There are some variants in the socket - basically the notches in a different place. But buy any normal stick-style M.2 drive and it will fit into the M.2 socket of your motherboard. It may not fit the length, though. The M.2 name tells you the size of it with the first two digits being the width and the last two digits being the length. So for example the Western Digital M.2 2280 SSD that I linked to above will be 22mm wide and 80mm long. 80 is kind of standard - you motherboard will probably fit that length. But you can see 110mm ones that might be too big for some board layouts. Check the specs on your board and it should say what M.2 devices will fit.
You will want a PCIe-v4 drive and likely NVME ('cause it's better so long as your motherboard is compatible). I looked up the X470 Prime Pro - this one, yes?
https://www.overclockers.co.uk/asus...ocket-am4-ddr4-atx-motherboard-mb-6c0-as.html
Looking at the specs half-way down it says:
1x M.2 (M-Key, PCIe 3.0 x4 & SATA6G with max. 32 Gbit/s, horizontal, Größen 2242 to 22110)
1x M.2 (M-Key, PCIe 3.0 x2 & SATA6G with max. 20 Gbit/s, horizontal, sizes 2242 to 2280)
So you can put in an PCIe 3.0 x4 up to 110mm long and a second one up to 80mm long. You'll want to put it in the first slot above because that runs at x4 and the other only runs at x2. That's why you want an PCIe 3.0
x4 M.2 drive - to get the most out of that slot. 1TB will cost you a bit even now, mind. NVMe is expensive!
Hope this helps. Samsung are currently the leaders for this sort of stuff. You could get this for example (although this one is "only" 500GB).
https://www.overclockers.co.uk/sams...-3.0-x4-nvme-solid-state-drive-hd-23n-sa.html
The terms you care about in its description are these: "PCI-E 3.0 x4" and "NVME". You'd care about the 80 part of "2280" as well, but your motherboard supports up to this length in both its slots so you're fine.