Life insurance policies generally have two beneficiaries to which their benefits will be paid. These are the main beneficiary and a second one to receive the benefits should the first beneficiary have already died before the benefits of the policy would be given out to the beneficiaries. There are two types of contingent beneficiaries: a revocable beneficiary and an irrevocable beneficiary.
Most policies would have a beneficiary and a contingent beneficiary, though not always. As mentioned, the contingent beneficiary is to receive the benefits of the policy should the first beneficiary already have died.
An irrevocable beneficiary cannot be changed.