The snippet directory contains snippets of HTML code (usually containing Liquid code, but it would also work with plain HTML data). This can be inserted in any of the templates (or even several times on the same page). By doing this, code duplication can be prevented (one of the largest origin of programming mistakes). If you think about using copy & paste for reusing a small part of HTML code, it will be best to create a snippet for it! The files in there can be of any name, just the extension need to be .liquid. There is a Liquid command for importing these snippets at any place in any template.