what is A file server and what is it for ?

Sharing files and folders across the network with Windows Server. We can specify shares with user access.

a file server is a computer responsible for the central storage and management of data files so that other computers on the same network can access the files.
A file server allows users to share information over a network without having to physically transfer files by external storage device

a file server is a computer attached to a network that has the primary purpose of providing a location for shared disk access

 A central location on your network where you can store and share files with users across your network. When users require an important file such as a project plan, they can access the file on the file server instead of having to pass the file between their separate computers. If your network users will need access to the same files and network-accessible applications, configure this computer as a file server.