Meta tag in HTML5

The meta tag should still be defined in HTML 5 like XHTML.

it’s generally used to describe a characteristic of a Web page, such as the character encoding. Character encoding is the internal representation of letters, numbers, and symbols in a file such as a Web page or other file that is stored on a computer and may be transmitted over the Internet. There are many different character encoding sets. However, it is common practice to use a character encoding set that is widely supported, such as utf-8, which is a form of Unicode. The below meta tag indicates
that the content of the document is a Web page that uses utf-8 character encoding.

<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>

although your page will continue to work without defining this tag, it better to define it to get a green ribbon on Validator!

Document Type Definition for XHTML

Because multiple versions and types of HTML and XHTML exist, the W3C recommends
identifying the type of markup language used in a Web page document. The
three types of XHTML 1.0, XHTML 1.0 Transitional, XHTML 1.0 Strict, and
XHTML 1.0 Frameset are used for XHTML but with HTML5 its simply : <!DOCTYPE HTML>