This is a library package that includes the PHP Markdown parser and its sibling PHP Markdown Extra with additional features.

"Markdown" is actually two things: a plain text markup syntax, and a software tool that converts the plain text markup to HTML.

The Markdown syntax allows you to write text naturally and format it without using HTML tags. More importantly: in Markdown format, your text stays enjoyable to read for a human being, and this is true enough that it makes a Markdown document publishable as-is, as plain text. If you are using text-formatted email, you already know some part of the syntax.