Deductible is what we pay towards the loss or damage, before the Insurance Company pays our Sum Assured.
OR
In an insurance policy, the amount paid by the policyholder in case of an accident before the company covers the loss is called deductible.
Deductibles are decided when the accident takes place

Example:
For example, you had a car accident and the cost of repair is 25,000 rupees. The insurance pays for most of the damage but you will have to pay some amount. depending on the policy, suppose you have to pay 3000 rupees. This is the deductible.
Insurance companies use deductibles to ensure policyholders have skin in the game and will share the cost of any claims.