This article has these four metadata tags:

Title: Minimal metadata
Slug: minimal-metadata
Tags: pelican, publishing
Summary: How little metadata can an article get away with?

The filepath used is /posts/2014/12/

How does Pelican know what to do with it?

Easy: the date is given as part of the filename. The slug and title are taken from the article.

To go really lightweight, remove tags and summary.