Markdown is a markup language (a language that uses symbols within the text for formatting, basically) that is commonly used in digital media due to its light weight and wide compatibility.
An example of a symbol that is used for formatting is the hash key (#). The hash key is used to denote a heading and the number of hash keys before some text defines the size and implied importance of the heading. Heading levels range from 1 to 6 with 1 being the biggest.
While both the asterisk (*) and the underscore (_) can be used to add bold and italic emphasis to text it is considered best practice to use underscores for italics and double asterisks for bold.
[link text](https://example.com)