A program can get inputs from, and send outputs to a special file. An application program can put its outputs into an ordinary file. Later it can easily be modified to put its outputs onto a console, or to ship them out a serial line to a remote site. The binary images of most commands are kept under this directory. More system commands and system configuration files.
Directory is initially empty.
You can place yourself on different nodes for convenience. To find out more about the ls command, look in the reference manual. Get a copy of the file, and put it under one of your directories. Now there is a two copies of that file on the disk. Build a link from one of your directories to the file you wish to share.