“NAS” is just an acronym for “Network Attached Storage”.

A NAS is literally any computer that is setup to host storage that’s accessible over a network. That’s it. Don’t get suckered into overpriced underpowered crap. Literally any PC made in the last decade has more horsepower than a brand new “dedicated NAS”. A Pentium G4560T (i.e. 6th/7th gen Intel) will run OMV or TrueNAS or whatever without a hitch. Stuff an old ATX case with hard drives, load OMV or TrueNAS or something, and go to town.