An absentee ballot is a special ballot which allows voters to cast their vote without going to the polls.
Each state has rules on who may request an absentee ballot. Some states allow any qualified voter to request such a ballot with no need to specify a reason, while others are very restrictive, such as being elderly, away from the state for various reasons (business, university, or in the military), or religious reasons.
There are basically two types of absentee ballots, a requested absentee ballot as defined by state statute law, and an unrequested absentee ballot, sometimes known as a mail-in ballot, used to promulgate voter fraud.
Conversely, some voters are concerned about ballots actually getting to the specified destination in time (if at all), and therefore will vote in person even if the voter would qualify for such a ballot.