Markdown is great, but it doesn't have any built in metadata abilities. Front Matter seems to be something of a semi-standard way of adding metadata to markdown docs. From what I can tell it originally came out of Jekyll. It's basically just a slab of fenced off YAML at the top of the file.
I'm personally a little against tampering with an open format like Markdown, but for my own personal notes and posts I'm not too concerned.
For parsing in JS, grey-matter seems to work well.