A "ballot" is the form that a person fills out when voting in an election. Ballots can be printed on paper, or now they can also be electronic.
An "absentee ballot" is a ballot that you fill out when you're not able to vote on the normal election day. In some countries, you can request to mail in an absentee ballot before the election instead of showing up on election day.
The word "absentee" comese from "absent", which means "not there".