A NAS is a "Network Attached Storage".
You simply plug it into your network. You can access it from all the computers. Most of the NAS have a printer switch, so you can also centralize your printer, making it available for all computers.
RAID stands for "Redundant Array of Independent Disks".
It's a standard for storage. In Industrial Services it's a common setup for storing the data. There are configurations with an unlimited number of drives available.
A RAID I has to have 2 drives, both are written identically. It's a good choice for home use.
I need to mention that a NAS is not a real backup. It can be destroyed (every occasion which destroys the storage box as a physical item) and the data on it can be deleted by all users, who have access.
But it is a fairly simple thing to use, and as real backups are nasty, time ripping and no fun at all, a NAS is a good choice to get some more storage reliability.
We do store so much today - photos, documents, music, memories, ... . It would be a shame to loose them.
regards
Torsten