Placed before all things, it is the first line of code of any html page. Neglecting the doctype, will prevent proper indexing and sometimes affect the readability of pages.
In summary, this is the "Document Type Definition". An indication of the computer language in which a page is coded, to allow web browsers to correctly interpret the code and provide proper rendering.
Indicates that: the HTML language (set by the W3C) is used in the DTD (DocTypeDefinition), it is version 4.0 of the language and the tags are shown in English (EN for English) and for now they only exist in this language ....
The head tags can normally provide a description of your site through keywords and phrases that describe your site. It has other features, like the automatic redirection to a URL...etc
The main tags found in the header includes the meta tags and link tags.
The Meta tags
Syntax: <meta name="keywords" content="keywords">
Maximum number of characters: 1000
Compatibility: All search engines.
Separate each word or phrase with a comma. You should also know that only the first 200-300 characters will are useful. (Other options: fr for French, en for English, en-us for American, ..etc)