A deductible is the fixed amount the policyholder pays out of pocket on each claim before the insurer covers the rest. For example, on a hull policy with a €300 deductible, €1,400 of damage means the insurer pays out €1,100 — the first €300 is yours.
The deductible is the lever that sets the price of the policy: the higher the deductible you accept, the cheaper the premium, but the more risk stays with you. Small damage below the deductible threshold is always entirely your own expense.