Front matter tells Jekyll to parse a file. You add predefined variables, which are YAML sets, to the front matter. Then, you can use Liquid tags in your files to access the front matter.
Front matter is indicated with two triple-dashed lines. You must place the variables between the two triple-dashed lines, and you must place front matter at the top of the file.
For example, the front matter for two posts in a blog about learning different musical instruments might look like this:
--- layout: post title: "Learning to Play the Violin by Self-Study" date: 2016-07-25 tags: [violin, self-study, beginner] ---
--- layout: post title: "Taking Lessons on the Violin as a Beginner" date: 2016-07-25 tags: [violin, lessons, beginner] ---
For more information about front matter, please see Jekyll Tips: Front Matter.