Geshi is a generic Syntax Highlighter. If you are not familiar with it you could find more information on the Geshi Webpage:http://qbnz.com/highlighter/

This Plugin give you some simple options to include a file from a specified directory into your content as a highlighted Syntax.

This is usefull for e.g. programmers to show codeparts or examples without needing them to write down in the WYSIWYG Editor of Joomla. I often had big difficulties embedding several codes directly in the article, time to time it cause to break my webpage design or simply broken the code itself. So this Plugin just use a very simple code which is fulle WYSIWYG Editor compatible.