"Jebem ti sunce", pronounced yeb-em tee suhn-tseh, means "I fuck your sun", but it is not actually used in a terribly disturbing fashion. I suppose it's because the common people in places where this curse is used say profane words pretty often (this includes my country just like Serbia).
This curse is often used when e.g. someone pulls a practical joke on you: even if they are your best friend you can tell them "Jebem ti sunce!" and it won't be taken to heart (unless they're hypersensitive, which is unlikely if they pull practical jokes on people).
There are several variations to this curse, like "Jebem li ti sunce da ti jebem sunce!" which is one of the more humourous ones. I'm not sure how to translate that, it's pointlessly circular.
Sometimes it's also used in anger, but there are plenty of other curses more appropriate for such situations. (I'd rather not go into details about those, there's enough swearing in this node already. :-)
Thanks to NatchLucid, who reminded me to write down how the phrase is actually pronounced.