From electowiki
Wikipedia has an article on:
parent: Help:Contents

This Help page (Help:Substitution or Help:Subst template or just Help:Subst) tries to describe how "subst" templates work. Except it does a really poor job. Go check out wikipedia:Help:Substitution, since that will probably be more helpful.

Subst-only templates

Some templates are really only designed to be used with the "subst" keyword. These are also known as Subst-only templates

Most templates get included like this:


But a subst template gets included like this:


What happens is that the generated wiki text replaces the template.

That's not a great explanation. Search for "subst-only" and "Wikipedia" using your favorite search engine, and you'll probably find a better explanation.