The template library is a tool designed to help theme designers and developers build great themes, and easily customisable plugins. It's purpose is to list all of the installed Templates in Moodle and show the documentation, as well as an example of the final output from each template in the current theme and language. A template is a small piece of HTML code that constructs one part of the Moodle user interface.
To use the tool, simply select a template from the list or search for a template and then click on it. The tool will show the documentation created by the template author as well as an example rendering of the template in the current theme.
This is useful for developers because it gives them a listing of all the templates that are available to use when designing new pages in Moodle. It is useful for theme designers, because it allows them to test all of the installed templates against a new theme to make sure that they display properly. It can also be used to ensure that templates display properly in "right to left" and "left to right" languages.