Airlines always try to overbook their flights, if they can. If you are not in a hurry to get to your destination, tell the airline personnel at check-in that you are willing to give up your seat if the flight is overbooked. Ask what kind of compensation you will receive for giving up your seat. Usually the airline will offer to put you in a hotel for the night, book you on the next available flight, provide you with meal vouchers, and give you a free plane ticket to and from your destination of choice.
This happened to me once when I was flying Virgin Atlantic from London to the US.