Most rental cars already include some sort of standard insurance called a Collision Damage Waiver or a Loss Damage Waiver. These waivers protect you from having to pay out the full value of the rental car if it gets damaged or stolen. So instead of paying the full amount, the rental company will charge you an excess.
This excess will be an out-of-pocket expense and can be thousands of dollars. If you want to avoid this, consider travel insurance with rental car excess insurance.