offset Line

offset Line

What is the 4th Line really doing 

  1. var posY = offset.top - $(window).scrollTop();

<script type="text/javascript">
    function show() {
    var offset = $(".logo").offset();
    var posY = offset.top - $(window).scrollTop();
    
     if(posY <=-80){
          $(".navbar-brand").css("display", "inline");
     }else{
         $(".navbar-brand").css("display", "none");
     }
}

show();
$(window).scroll(show);
</script>