Travel insurance for cruises is intended to protect you from hardship. It typically reimburses you in some way if your trip is affected by unexpected events, such as dockworker strikes, death of a loved one, delayed plane flight, or lost luggage. Also usually covered are medical emergencies during the cruise, including evacuation from the ship.
Acts of God, such as hurricanes or earthquakes are usually not covered by cruise travel insurance.
The insurance is available from third parties or from your cruise line. Third party providers can cover you in the unlikely event that the cruise line goes bankrupt.
Read the fine print of your cruise travel insurance before you buy. Policies differ on what is covered and how you'll be reimbursed. Cruise Critic has more information on travel insurance for cruises.