If you have protected no claims and you, say scratched your car on a wall, would your insurance cover the cost, or do you have to pay something towards it?
If you had a comprehensive policy, then your insurance would cover it. But you would have to pay any excess: almost everyone has an excess, usually more than a scratch would cost to repair.
If you had protected NCD then such a claim wouldnt affect your NCD, but you'd have a fault claim on your record which would probably increase your base premium.