An absentee ballot is cast when the voter cannot (or does not wish to) physically attend their allocated polling station to vote.
In the United Kingdom, we have access to postal votes, meaning that citizens of the UK that are eligible to vote may do so from anywhere in the world (providing that their postal vote will arrive in time to be counted). To vote by post, you must fill out and sign a form, which then must be returned to your local election registration office in advance of the election.
Alternatively, if you are not able to attend your local polling station, you may cast a proxy vote. This involves choosing someone (who is over 18 and registered to vote in the UK) to vote on your behalf. As with the postal vote, your decision to vote by proxy must be declared in advance.