This vector defines a variable or fetches the value of the variable, depending on usage.  If you are using it to fetch a value, the vector itself will be replaced by whatever value is assigned to it at that time.  If a variable called "first_name" has a value of John, then "{{var:first_name}}" would become "John".

Examples

Assigning a value to a variable
{{var:first_name=John}}
{{var:header=<h1>Welcome to my website</h1>}}

Fetching the value of a variable
{{var:first_name}}
{{var:header}}