It means you're trying to access an index in an array that doesn't exist.
For example, if you said this:
if($_POST['username'] && $_POST['password']) {
check_login();
}
but the user hasn't submitted the login form yet, you would see such an error, because you're trying to access an array item that doesn't exist.
Instead, the above code should be written:
if( isset($_POST['username']) && isset($_POST['password']) ) {
check_login();
}
