I assume you are referring to InsureMyTrip.
This is a great website where you can compare policies and make an educated decision.
How does it work? You simply select a policy which covers the cost of your cruise, air fare, luggage, medical emergencies, etc., and buy it. There are many caveats though, such as pre-existing illnesses not being covered unless you buy the insurance within X days of booking your cruise. This can be 7 to 14 days depending on the insurer.
I always get insurance, and generally speaking the insurance offered by the cruise lines is not as good - nor as cheap - as you can get from a regular insurance company. I have personal knowledge of many sad stories where people didn't buy insurance and then something happened just days before the cruise and they lost all their money.