An insurance deductible is a way for the insurance company to share a risk with the policyholder, and to reduce the premium payment required. By buying a policy with a $500 deductible, the policyholder agrees to be responsible for the first $500 of any covered loss. The insurance company is insuring only those losses exceeding $500, so they charge less for the premium. Most claims will be less than $500, or not much over, so their risk is reduced.
You can usually buy a policy with a smaller deductible, or with no deductible at all. You will find those policies are considerably more expensive, as you're asking the insurance company to assume a greater risk. The general rule is that your deductible should be the largest amount you could cover with your own funds, should a loss occur. If you buy a collision policy for your car with a $500 deductible, and the car is destroyed, it will cost you $500 to replace it--the insurance company will pay the rest. So long as you can lay hands on $500, you know you'll have a car to drive. If you get through the policy term without the car being wrecked, you get to keep your $500, and the money you didn't pay for a higher premium with less risk.