What is meant by application hardening?

What is meant by the term "application hardening?" How do I get started hardening my apps?
Application hardening is a process to changing the default application configuration in order to achieve greater security. For example, one can configure the Apache Web server in a more secure manner by modifying the httpd.conf file.

Editor's note: One place to learn more about application hardening is in this free chapter from Hardening Linux.