A template is a predefined set of wiki pages. When a template is set, the wiki starts off with the content defined in the template.
The template applies to each subwiki; in 'One wiki per group' mode, for example, each group's wiki is initialised with the pages in the template.
To create a template, write the pages you want on any wiki, then visit the Index page and click the 'Save wiki as template' button. (You can also manually create templates in other software; it is an extremely simple XML format. Look at a saved template to see the format.)
You cannot change the template after the wiki has been created. If you want to do this, delete the wiki entirely, then create a new one using the template.