What is the Doctype ?

The Doctype is an indication in an HTML page to define the type of syntax used in the page. 

Why is it important ?

If the Doctype is not correctly defined, the browser may have difficulty displaying your page and display it in an unexpected way. This is called the Quirks mode of a browser.

How do I set the Doctype ?

Just add the "<!DOCTYPE html>" insctruction at the top of the HTML document : 

<!DOCTYPE html>
<html lang="en">

It is recommended to use the HTML 5 doctype, the other versions are considered "outdate".