As a starting point, NAS = Network Attached Storage. Basically, it's a drive that connects to your network rather than to a specific computer. That's extra convenient if you have multiple computers you want to share files between.

NAS will usually support multiple drives in RAID which is Redundant Array of Inexpensive Disks. A reasonable starting point is a unit that has two hard drives configured as RAID 1, which just means that the two drives always have the same content. If one of those drives fails, the NAS will flash some error lights (and there are other ways of getting those messages) and let you know. You can then swap out the bad drive and it will automatically copy the data to the new drive. So you never lost any data and you can reestablish that level of safety.

There are various bells and whistles, like being able to connect to the drive from anywhere on the internet, but personally I don't use any of those things, I'm just using it as a backup and to move files around. (Oh, also I have all my music there to be played by Sonos.) Synology has a good reputation though.

You can of course backup files from your computer to a NAS. That's not cold storage because the drive is still accessible, which is good and bad; it makes it easier to make frequent backups, but if something malicious happens (rather than simply a drive failure), it could also affect any attached drives. Those sorts of trade-offs are why there is the 3-2-1 sort of guidance.