In an American election, the ballot paper lists the official candidates with a box beside each name — but there is usually one blank line at the bottom where you can write any name you like. A write-in is a vote cast that way, and by extension the candidate who gets such votes: "a write-in vote", "a write-in candidate", or just "He ran as a write-in". This is very much an American word; if you are talking about Taiwan's elections, or Britain's, there is nothing to translate, because their ballots do not have that blank line. Most write-in votes are protest votes or jokes — people write in a cartoon character, a dead celebrity, their own dog — and they are counted but almost never win. That makes the rare exception famous: a write-in candidate actually winning a US Senate seat is the kind of thing people remember for decades. Note the hyphen and the stress: WRITE-in, with the weight on the first part, because it is a noun made from the two-word verb "write in". Compare the verb: "I wrote in her name" (stress on "in"). It is also used loosely for any written suggestion sent in by a member of the public, but the election meaning is the main one.