You might be asking what a NAS is right now -it is a tool. A NAS is a network attached storage, and it is used to provide reliable storage for files. It usually contains some form of disk redundancy so that if a disk (or two or three) dies, you don’t lose anything.
For the record, I have the Synology 1019+, although it appears as if it has been replaced by the 1520+ https://www.synology.com/en-us/products/DS1520+.
This product, plus the drives, cost me $1500 or so — about the cost of a computer, and one I did not want to spend at first. Since that day though, it has provided peace of mind and kept my files safe in many ways.