As covered in Your First Page, the front matter is the stuff enclosed in 3 dashes at the top of the page. The front matter is used to configure the settings specific to that page, such as the title and permalink. Certain pages like the home page and contact us page have more configuration options, such as the content in the hero banner or the phone numbers displayed.
The way the front matter is structured is called YAML. You can think of it as a language used to specify data in a way that can be read and understoof by Isomer.