This example looks at the "standard module format" used by many components in YUI to represent modular content on the page. The Module Control is a JavaScript representation of a standard module, one that provides some basic plumbing for interacting with the head, body and foot sections of a standard module. Convenience methods for showing, hinding and rendering the module are also included in the Module Control.
You're most likely to use the Module Control when building a custom widget; its importance in the YUI Library is as a foundation rather than as a UI control in its own right.