No Claims Bonus (NCB) are designed to reward careful drivers, which in effect means those drivers who do not make a claim on their insurance.

For every year that you have a car insurance policy and do not make a claim on it you will receive 1 years No Claims Bonus (NCB). These No Claims Bonus then build up and the more you have the lower your insurance is likely to be when you come to renew.
Insurance companies tend to view 5 years NCB as Full No Claims.

Protected No Claims Bonus
Protected No Claims are for drivers who have 4 years minimum No Claims Bonus. What protected No Claims does is mean that if you do have an accident or need to make a claim then your no claims bonus will not all be lost. You do pay extra to protect your no claims though.
However you can't make lot sof claims in a year and expect to keep them, you should check with your insurance company how many claims you can make without losing your no claims.