What does the title of the anime "K" mean?
There are two possibilities for what the "K" refers to.

The first is Adolf K. Weismann, who is the original person that became Yashiro Isana, the main character of the show.

The second is "King". The leader of each clan is its King, who controls the members of the clan.  Weismann is a King.

KINGS is also the first opening theme of the show.