Most policies offered by German private insurers also have a deductible. This is the amount you have to pay out-of-pocket before the insurance kicks in. Let's say your plan has a €500 deductible. This means that each year, you will have to pay your first €500 in health costs out of pocket. After that, you will be reimbursed according to your policy.
A deductible is used to encourage you to be aware of your health costs, and a higher deductible generally results in a lower monthly premium. For people who tend to go to the doctor frequently, a lower deductible is recommended.