A deductible is the amount of money deducted from any claims payout made against the policy and is either expressed as a $ figure or as a % of the total value of the insured item.  

Example A

You have a deductible of $500 on your comprehensive car insurance.  An accident occurs and the cost to repair your vehicle is $2,500. You are responsible for $500 while Indigo pays $2,000.

Example B

Your home is insured for $250,000 with a 3% deductible for catastrophic events (hurricane, earthquake etc).  A hurricane damages your home. You are responsible for the first $7,500 of damage while Indigo covers the difference.