"Network addressable storage" is storage that's available to any PC, tablet or other device on a network. It's normally a hard disk or group of disks, but can use solid state drives as well. A NAS has its own processor to manage the storage. To each connected device, it looks like a directly-connected storage device (hard disk or thumb drive).
Most Lenovo NAS systems include RAID capabilities, so your data is protected against the failure of one (and sometimes more) hard disks.
A NAS is a server, but it's focused on sharing files between computers and doesn't have other server functions like email. The operating system and other software is built in, so you can simply plug the NAS in and get it running with minimal setup.
A NAS is a great way to provide centralised backup for all your PCs and to share large volumes of data like photos, music, movies or TV shows at home. In a business they're great for all your business data and for video surveillance. Just tuck it away in a corner of your home or business premises.
RAID stands for "redundant array of independent [or inexpensive] disks". It's a collection of hard disks (or other compatible storage devices), working as a group, in most cases* with data spread across the disks so it's protected even if one disk fails ("redundant" in a technical sense). Sharing the data across several drives can also speed up data transfer.
To a connected computer, a RAID array looks like one big disk, though it may be divided into several volumes.
There are several "levels" of RAID, which distribute the data differently and give priority to speed or data security. Briefly, they are:
RAID 0: This 'stripes' data across two drives which speeds up data transfer, BUT has no redundancy. If one drive fails, all data is potentially lost.
RAID 1: This 'mirrors' data to two or more pairs of drives. It will continue to work if one drive in a pair fails.
RAID 5: A minimum of three disks, with data (and 'parity' data) distributed across all disks. Any disk can fail without losing data.
RAID 6: This needs at least four disks, and can maintain data even if two drives fail.
RAID 10: This is a combination of RAID 0 (striping data) and RAID 1 (mirroring). It can tolerate a failure of one drive in each RAID 1 'span'.
The amount of available storage varies between RAID levels. Only RAID 0 gives you the total combined space of the drives. RAID 1 gives you half the total. RAID 5, 6 and 10 give you somewhere in between, depending on how many drives are in the array.
Wondering what happened to RAID 2, 3 and 4? They're older implementations that are rarely seen now.