Unlike hiring a car in Australia or Europe, where liability is limited, the car-hire game plays by different rules in North America.
If you have an accident and damage your vehicle, you may be liable for the entire replacement cost.
You may also be liable for damage that your vehicle causes to persons or property and to cover both you need collision damage waiver (CDW) and loss damage waiver (LDW).
One way around this is to accept the full insurance package that your hirer will offer when you front up to their desk, but it's expensive.
For example, Hertz charges $40 for CDW and $60 for LDW, per day.
You'll save massively if you book your hire vehicle through an agency that offers an all-inclusive price, with CDW and LDW in the package.
Google "car hire USA" and you'll find several such agencies.
You also need additional liability insurance (ALI), which insures you against claims made by third parties for injury or death and property damage caused by your rental vehicle.
This is often included in your travel insurance policy.
Some credit cards offer cardholders who book using their credit card a level of protection that will offset the need for CDW/LDW but this privilege is generally confined to upper-tier cards with high annual fees.
As with all vehicle insurance, you may void your cover if the accident was caused by you speeding, driving under the influence or driving recklessly.