Define keywords for search engines:

<meta name="keywords" content="HTML, CSS, JavaScript">
Define a description of your web page:

<meta name="description" content="Free Web tutorials for HTML and CSS">
Define the author of a page:

<meta name="author" content="John Doe">
Refresh document every 30 seconds:

<meta http-equiv="refresh" content="30">
Setting the viewport to make your website look good on all devices:

<meta name="viewport" content="width=device-width, initial-scale=1.0">