The italian version of the middle finger (F*** you).
Put your hand on the middle of the other arm and bend that arm a little.
It's called Umbrella because your arm obtain the shape of an old umbrella handle.
It literally means what I said in the beggining:
It's maybe the most rude gesture of our repertory, and I suggest to avoid using it in almost any occasion, or your vacations in Italy can end badly XD