This is a negation pseudo-class selector. This takes simple selector as an argument and selects elements that are not represented by the argument.
Example:
<!DOCTYPE html>
<html>
<head>
<title>CSS pseudo-class selector - :not</title>
<style>
p:not(#pFour) {
color:red;
}
</style>
</head>
<body>
<p>First para</p>
<p>Second para</p>
<p>Third para</p>
<p id="pFour">Fourth para</p>
</body>
</html>
Result:
Above example sets text color RED to all P elements except P element with ID equal to “pFour”.