A payable-on-death account is a bank or brokerage account with a designated beneficiary. Sometimes these are also called "Totten Trusts" or transfer-on-death accounts.
Whatever they're called, at the death of the account owner, the assets in the account are distributed to the person or people designated as account beneficiaries.
Usually, all that's required at the death of the account owner, is for the designated beneficiary to fill out a claim form and to supply a copy of the death certificate.
If you're not sure whether or not an account had a designated beneficiary, you'll need to check with the bank or brokerage company and check on the account's registration.
For example, if Nila opened a checking account and then designated her brother, Jack, as the payable-on-death beneficiary, upon Nila's death, Jack will receive the account's assets.