A Dirndl is a typical German dress originally from South Germany. It's normally known worldwide because of the Oktoberfest in Munich. The Dirndl can also be worn for a wedding. The dress is very colorful and tight. Most of them are very exclusive and therefore very expensive (100€-400€). Each Dirndl has a ribbon and depending on where you wear this ribbon (left side, right side, front side, back side) it means something different:
This is some kind of “guide” for men to know if they should approach a woman or not. Of course, this is an old tradition, which some people nowadays consider sexist. But it's still used by some people.