$(this).filter(".descBox").show(500);
Usually you use filter to do slightly more complicated things. For example if you changed the background of all the divs inside "this" parent, and then you want to add a border to only the "descBox" class within all the divs with descBox classes inside "this".
Something like this (essentially lifted from the manual):
 $("div", this).css("background", "#c8ebcc")
               .filter(".descBox")
               .css("border-color", "red"); 
Maybe in this context:
<div>
    <div></div>
    <div class="descBox"></div>
    <div class="descBox"></div>
    <div class="descBox"></div>
    <div class="descBox"></div>
    <div></div>
</div>
<script>
$("div").click(function() 
{    
    $("div", this).css("background", "#c8ebcc")
                  .filter(".middle")
                  .css("border-color", "red");
});
</script>