Dim background elements when displaying submenu
There's a bunch of options.  jQuery is probably the easiest but I could envision a way of doing it with CSS only using some combination of clever sibling and pseudo-class selectors.  You could also do it using an overlay element and some z-index work. Or just use JS/jQuery to dim the other element's opacity either using animate or adding a class that has an opacity transition.  The best choice depends on your requirements and how your HTML is structured.

But the question is a little vague and not particularly Drupal related.  If you want do the last case above, just do on('hover', function() ) {} and set the opacity / class of the elements to dim.