Rental car agencies offer collision damage waivers and liability policies. The collision damage waiver isn't insurance. It is an agreement that the rental company won't, with certain exceptions, try to make you pay for any damage to a car you rent.
If you have auto insurance, your policy may already cover damage to a rental car, but the coverage might be less than the value of a rental car. Read your policy to know what's covered and the coverage limits. If your coverage limit is too low, consider increasing it. You will pay more in premium, but it might be cheaper than buying additional coverage through the rental agency, especially if you rent cars often. If you aren't sure if your policy covers damage to a rental car, ask your agent before you rent the car.
If you don't own a car, but borrow or rent cars often, you can buy a non-owner liability policy. A non-owner policy pays for damages and injuries you cause when driving a borrowed or rented car, but it doesn't pay for your injuries or damage to the car you are driving.