The Merriam Webster's Collegiate Dictionary defines security in a number of ways. There are two definitions which are of particular interest to you here:
The quality or state of being secure as freedom from danger.
Measures taken to guard against espionage or sabotage, crime, attack, or escape.
When network security is discussed, certainly the first point applies: You want to be free from any perceived threat. Secondly, you must guard your system resources from being examined without permission, sabotaged in some manner, stolen, or otherwise attacked. Your written works that are not public must not "escape" from your system without permission.
The complexity of this task and the broadness of its application make it impossible to fully treat this subject in one chapter. However, some aspects of network security apply directly to socket programming, and you should not ignore the dangers that exist. A few simple measures presented in this chapter will help prevent your servers from being exploited or attacked. You will learn where vulnerabilities exist and what you can do about them.