A deductible is an amount that you as policyholder pay as a lump sum in the event of a claim, any excess cost will then be payed by the insurer. Including a deductible in your policy results in a substantially lower insurance premium. As a very general rule of thumb, for partially comprehensive (Teilkasko) you should choose €150 deductible, for fully comprehensive coverage (Vollkasko) you should choose €300 to get attractive insurance rates.
Imagine you have a damage on your own vehicle.
The repair costs are 800 €. If you have chosen a €150 deductible, you pay the first €150 yourself and your insurance pays the remaining 650 € of the costs of the damage.