HTML element: meta

The <meta> tag must specify exactly one of the following attributes

name,
http-equiv,
charset or
itemprop:

name=

<meta name="…" content="…" /> is used to include page-wide metadata.

The name attribute can have one of the following values:

author
description
generator
keywords: a set of comma separated keywords
referrer: a referrer policy, one of "", "no-referrer", "no-referrer-when-downgrade", "same-origin", "origin", "strict-origin", "origin-when-cross-origin", "strict-origin-when-cross-origin", "unsafe-url"
color-scheme (for example <meta name="color-scheme" content="dark">
theme-color
robots

A web page author is free to add other names, too.

Google does not honor the description and keywords attribute anymore in order to rank a page.

name="robots" for search engines and bots

With name="robots", all search engines are addressed. In order to address Google only, use <meta name="googlebot" …>.

<meta name="robots" content="noindex"   /> <!-- Don't index the page                                -->
<meta name="robots" content="nofollow"  /> <!-- Don't follow links on the page                      -->
<meta name="robots" content="nosnippet" /> <!-- Don't show a text snippet in the search result pane -->
<meta name="robots" content="noarchive" /> <!-- Don't archive the content of the page, no cache.    -->

Multiple options can be joined with a comma

<meta name="robots" content="noindex,nosnippet,noarchive" />

name="viewport" for mobile devices

See <meta name="viewport" …>

http-equiv=

The http-equiv attribute can have one of the following values:

content-language (Compare to <html lang="…">)
content-security-policy
content-type, see Specifying a characterset within an HTML document
default-style
refresh, See Redirection with HTML
set-cookie (should be ignored by user agents)
x-ua-compatible
Accept-CH (Compare with the Accept-CH response header)

In HTML 5, <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> can be replaced with:

<meta charset="utf-8">

http-equiv=x-ua-compatible

The following example instructs Internet Explorer to display a webpage in IE9 Standards mode. Thus, the website can only use the features that are offered by IE 9.

<meta http-equiv="x-ua-compatible" content="IE=9">

The interpretation of x-ua-compatible depends on the web browser that renders a HTML document.

If a web page has both, a <!DOCTYPE> declaration and a x-ua-compatible, the x-ua-compatible overrules the doctype declaration. (At least in IE).

OpenGraph properties

See: https://developers.facebook.com/docs/sharing/opengraph

<meta property="og:title"       content="Add a headline here." />
<meta property="og:description" content="Describes the page with up to 297 characters." />
<meta property="og:type"        content="article" /> <!-- or blog, or website, or …
<meta property="og:image"       content="https://foo.bar/path/to/image/shown/image/that/facebook/shows/as/screenshot.jpg" />
<meta property="og:url"         content="…" />

Apparently, OpenGraph properties can be debugged with the FaceBook OpenGraph debugger.

Some examples

<meta name="Generator" content="The very cool CMS application">

<meta charset="utf-8">

<meta name="robots" content="index,follow">

<meta name="google-site-verification" content="sfRzmUzt…">