A more thorough explanation:

A provisional ballot is a type of paper ballot used by a voter when there is an issue establishing their voting eligibility. It is only counted after the issue is resolved and the voter is deemed eligible to vote by election officials. This type of ballot is also known as an “affidavit ballot” in some states.

Each state has its own provisional ballot statute and verification process. The process usually involves local election officials reviewing government records or asking the voter for more information, such as photo identification not presented at the polling place or proof of residence. In nearly all of the states, cast provisional ballots are kept separate from other ballots until after the election, at which point they are either verified or denied, and consequently counted or discarded.

The Help America Vote Act of 2002 guarantees that, in most states, a voter can cast a provisional ballot if they believe they are entitled to vote. Common reasons for casting a provisional ballot include:

• The voter’s name does not appear on the precinct register and the voter’s eligibility is unverified (shown as either not registered to vote or registered to vote elsewhere).
• The voter refutes the supervisor of elections’ office confirmation that they are not registered or eligible to vote.
• There is an indication on the precinct register that the voter has requested a vote-by-mail- ballot (absentee ballot), the voter does not have a ballot to submit, and the poll worker is unable to verify whether they have voted.
• There is an indication on the precinct register that the voter has returned the vote by mail ballot or has voted in the office or at an early site, but the voter maintains that they have not voted, even after a call to the Supervisor of Elections office.
• The voter did not provide proper picture/signature identification.
• There is indication on precinct register that the voter’s driver’s license, identification card, or Social Security number is not yet verified by the state.
• The voter’s registration contains inaccurate or outdated information such as the wrong address or a misspelled name.
• The voter’s party registration is listed incorrectly and they wish to vote in a closed primary.

For example, if a voter goes to their polling place and their name is not on the list of registered voters, they may be given a provisional ballot to fill out. The election officials will then review the voter's information to determine if they are eligible to vote. If they are, the provisional ballot will be counted along with the other ballots. If they are not eligible, the provisional ballot will be discarded.