Declare the Doctype of an HTML Document

Tell us what’s happening:
I don’t understand what it means
help me please
I am bad in English

Your code so far

<!DOCTYPE html>


Your browser information:

User Agent is: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.181 Safari/537.36.

Link to the challenge:

I got this guys