It refers to the Document Type Definition. It states the rules for the Markup Language for the Browser to provide the Content properly. The DOCTYPE is not required in HTML5 currently. Nevertheless, it should be placed prior to the HTML Tag
i.e., the first line of any HTML Document. It is not an HTML Tag yet a direction to the Web Browser that shows the Version of the Markup Language that the Web Page is composed in.

 Example : 

 < ! DOCTYPE html >