A provisional ballot is a ballot used to record the vote of an individual whose eligibility to vote cannot be confirmed at the polling location on election day by election officials.
If the Officers of Election cannot find your name in the pollbook or there is some other question regarding your eligibility to vote (such as you received an absentee/vote-by-mail ballot). A provisional ballot is designed to protect your right to vote by allowing you to submit a ballot that will be counted if the underlying question of your eligibility can be resolved.
Additionally, if you go to your polling place and do not have proper ID, you will have to vote a provisional ballot. Follow the instructions given to you by the Officers of Election so that you can provide your ID by the deadline, and your vote will be counted.