What is "write in"

vb. 1 (context intransitive English) To write a letter to, e.g. a publication. 2 (context transitive English) To fill in something required, by writing.

1. v. cast a vote by inserting a name that does not appear on the ballot; "Many voters wrote in the names of strangers"

2. write to an organization; "Many listeners wrote in after the controversial radio program"