What is NAS storage?

Network Attached Storage. The Word "storage" in the question is redundant, as the S in NAS is "Storage" It's an enclosure holding a large amount of RAID disk storage, which is directly attached to the network. The NAS does not need a decicated computer system to run it, as the enclosure contains the network interface card (or cards if it is connected to multiple network ports), and all of the required control circuitry.