You mean isset($_POST['posted'])
The isset function is used to see if a variable has been declared, and is not NULL. If you try to use a variable which has not been declared PHP will generate a warning. See www.php.net/isset
for a full description.
It is most often used to check that the input to the page ($_GET, $_POST, $_SESSION etc) has specific data set, for example if you have a page which is passed an ID in the querystring, you should check to see if the ID is present, so you can show your own error message or something.
$productid = $_GET['id'];
// ... use the id to show details ...
print "Invalid Product ID";
Hope this helps