A no claims discount is a reduction that drivers can get on car insurance if they have no made a claim. It is often on a sliding scale with more savings to be made depending on how many years the driver has not made a claim for. If you make a claim then normally you will lose that no claims discount, however some circumstances may be exempt such as vandalism , broken windscreen or accidents which were not your fault and you were able to claim on the other drivers insurance.
It is possible to take your no claims discount with you when you move insurance companies which means that you can benefit from a discount even with a new insurer.
If you do have an accident you may not be able to keep your no claims bonus even if it was not your fault. It depends on whether you have had to make a claim on your own insurance. If it was a theft, fire or hit and run then it is unlikely to be possible to find out who did it and claim on their insurance.
You can pay extra to protect your no claims bonus should you have an accident This means that although your premium will go up as a result of the accident, you will not have to lose the percentage discount that you get form the no claims bonus. It is questionable as to whether it is worth paying for this or not and probably depends on whether you will make significant savings or not if your no claims is saved. The longer you have been without making a claim, the better the discount you will receive and the more worthwhile it will be to protect it. It is something worth thinking about, comparing quotes for and then deciding whether or not it is worth it for you to take out.