[jQuery] help read code

[runrunforest]

Hi,

this code is from a dropdown box, where clicking on the "a.signin"
will open or close the box. So I don't really get how the syntax below
work

                        $(document).mouseup(function(e) {
                                if($(e.target).parent("a.signin").length==0) {
                                        $("fieldset#signin_menu").hide();
                                }
                        });