The no-claims bonus is a reward for every full insurance year without a claim.
The SF class (Schadenfreiheitsklasse) is downgraded after a claim is settled. If this also reduces the no-claims bonus, your insurance premium may increase. Depending on the respective insurer, it is possible to agree a no claims discount protection.
A so-called bonus-malus is that it encourages you to drive more cautiously and avoid accidents (which, in turn, positively affects traffic safety statistics).