The NAS (Network Attached Storage) is a type of computer that, in general, has a single function. It contains some form of data storage devices such as disk drives or SSD devices. The primary purpose of a NAS is to make the information stored on the drives available to other computers on the network through file sharing. Ordinarily, the user interface is via a web browser.
Over time the operating system and applications on NAS computers have gotten more powerful. Companies such as Synology and Qnap have developed many tools in their NAS devices to allow them to function on many more roles that the file server. In audio applications, the NAS is principally used to store large quantities of music files. Also, the NAS can have software applications installed to act as a music server and more.
There is more information on Wikipedia: