Pair tag have a closing at the end eg: <b> text </b> Impair tag does not have closing
Tags that do not have a closing tag will probably not render. If it's somthing like the <html> tag or the <body> tag, the whole web page may not work.
</body>
An opening tag is what shows the HTML that it is starting there, a closing tag shows when to stop whatever the opening tag said.
It is an acronym for several things, see the link below.