When a page gets rendered, it goes through a template engine that can expand special syntax that runs logic. This is useful for authoring pages that list blog posts by date or category, for instance.
Here is a list of the template engines that come with PieCrust out of the box.
Jinja2 is a powerful template engine that is set by default in PieCrust. You can read the templating documentation here.
Mustache is a minimal template engine that runs fast and keeps your templates files simple and clean.