An insurance card is a piece of paper that you are supposed to keep in/on/with your vehicle that has the name of your insurance agency and your account number so that if you get pulled over by the police they can verify that you actually have insurance if you are in a state that requires motor vehicles to be insured. Check with your local DMV to find out for sure.
Other types of insurance cards exist for other reasons, all relating to proving that you carry some or another type of insurance you may need.