The :target pseudo-class represents a unique element, with an id matching the fragment identifier of the URL of the document..
For example, the URL http://website-path#demo contains the fragment identifier #demo. In HTML, identifiers are found as the values of either id or name attributes, since they two share the same namespace. Thus, the example URL would point to the heading "demo" in this document. - MDN
Its a very powerful and useful pseudo-class you can use it as tabbed menu, multi popup etc.
Example :-
CSS -
:target {color: red;border: 1px solid #ccc;}
HTML -
<p id="one">Section ONE</p>
<p id="two">Section TWo</p>
<p id="three">Section Three</p>
<a href="#one">First</a>
<a href="#two">Second</a>
<a href="#three">Third</a>
Output -
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