HTML head tag

The head tag is metadata container. It contains the title of the page, inner CSS or JavaScript content, external links for CSS and/or JavaScript and some metadata about the page like the encoding...

meta tags meta value
meta description This a meta tag with a summary of the page content. It is used by google by displaying the description under the result link. For sample <meta name="description" content="My super page about HTML">
meta content-language It will specify the language of the page like en for english or de for deutch.. When this meta is missing, it means your page content is for all languages. The language can also be set using the lang attribute of the html tag.
meta content-type This is a meta tag for insicating the document mime type (like text/html) and the text encoding (like charset=UTF-8). For sample <meta http-equiv="content-type" content="text/html;charset=UTF-8">
meta X-UA-Compatible This is a meta for helping the navigator working in a good mode. This is mainly useful with IE than can be configurated for working with various versions (like IE8,IE9...). For sample <meta http-equiv="X-UA-Compatible" content="IE=Edge,chrome=1"> will force IE working with the last version.
meta viewport It indicates to the navigator how it must fit the page. If you want to have the page fully without initial zoom, then you may put <meta name="viewport" content="height=device-height,width=device-width,initial-scale=1.0,maximum-scale=1.0">
meta robots It indicates to the seach engine bot how to the process the page. By default a bot will scan the content and will index the links. noindex will mean don't process the content. nofollow will mean don't index the links. none mean to the bot to ignore the page. all is a default value processing the content and the links.