Pandoc is a command-line tool for converting files from one markup language to another. Markup languages use tags to annotate sections of a document. Commonly used markup languages include Markdown, ReStructuredText, HTML, LaTex, ePub, and Microsoft Word DOCX.
In plain English, Pandoc allows you to convert a bunch of files from one markup language into another one. Typical examples include converting a Markdown file into a presentation, LaTeX, PDF, or even ePub.