<!DOCTYPE html> — The doctype. In the mists of time, when HTML was young (around 1991/92), doctypes were meant to act as links to a set of rules that the HTML page had to follow to be considered good HTML, which could mean automatic error checking and other useful things. However, these days no one cares about them, and they are just a historical artifact that needs to be included for everything to work right. For now, that's all you need to know.