Networking is the act of connecting two or more computers so that the can communicate with each other. The actual methods of connecting and communicating are slightly complicated, but the end result is very attractive.
UNIX operating systems have many networking features. Most basic services--filesystems, printing, backups, etc--can be done over the network. This can make system administration easier, since it allows centralized administration, while still reaping in the benefits of microcomputing and distributed computing, such as lower costs and better fault tolerance.
However, this book merely glances at networking; see the Linux Network Administrators' Guide for more information, including a basic descriptions of how networks operate.