What does AD and D limit means in insurance?

It might be different for you, but with my insurance AD&D stands for Accidental Death and Dismemberment, and the limits are the maximum amount of compensation that can be distributed to you upon fulfillment of the policy.

Accidental Death and Dismemberment: A certain amount would be paid if death was result of an accident, rather than a disease, for example. Dismemberment would be if somebody lost a hand (or use of the hand) or foot or leg, etc.