It is a Tomcat specific application that helps in the automated deployment, starting and stopping for web applications.
This is useful as part of our ant build scripts.
This is worth setting up on your development machine. To setup, go to your local running copy of Tomcat and click on the Tomcat Documentation link, then the Manager App HOW-TO link.
The page tells you how to set up the manager. If you've setup 4.x with default settings, the link is http://localhost:8080/tomcat-docs/manager-howto.html