It is an accidental death clause. The accidental death benefit is paid in addition to the death benefit, should the insured's death occur due to an accident. In double indemnity, twice the face value of the policy will be paid to the beneficiary; in triple indemnity, three times the face value is payable.
Accidental death caused by war, aviation except as a passenger on a regularly scheduled airline, and illegal activities is generally excluded. Time and age limits are usually applicable, as for example, the insured must die within 90 days of the accident and be age 60 or less.