I know that Greece is holding a crucial referendum this weekend. But beyond that I am really confused. What's the vote about? 

You're not alone. 

But to answer your question, it's probably best to back up a little bit. 

For months the Greek government has been negotiating with its creditors. Greece wants to get its hands on the last of its bail-out funds and for the country's debts to be reduced. The creditors have said that Greece can only have the money it needs if it makes a number of reforms to its economy (which boil down to raising taxes and cutting spending) but debt relief is a no-no.