If Prime Minister Renzi loses then it is likely we will see Italian banking and finance issues. Such events always spill over to the uk. If he then has to resign then we potentially could see another Southern European country in trouble. And we suffer every time one of our neighbours economy struggles - not least because the Italians will not be buying our exports.
The Bbc take on the story is here: http://www.bbc.co.uk/news/world-europe-38198177