The title seems pretty straightforward on the surface. It's about a guy (in this case, a king) named Cymbeline.
Or is it?
We're not so sure this is really Cymbeline's story. He's not exactly the protagonist, and his wife and daughter have much stronger presences in the play. So why isn't the play called Imogen, then?
It could be because Shakespeare doesn't name his plays after chicks. The best they ever get is to share a title with a guy (Romeo and Juliet, Antony and Cleopatra, Troilus and Cressida), and even then they never get top billing.
It could also be because if there's a king in the play, Shakespeare usually names the whole thing after him, even the king in question isn't really the main character. Take Henry IVor Henry VI, for example. Both of those plays are named after the kings in them, even though a lot of people say these plays are about a bunch of characters finding their way in life... or taking over a kingdom (or two). They're not really about the kings themselves.
The title of this play also clues us in to the fact that the genre is all over the place. When the play was originally published in the First Folio in 1623, it was titled The Tragedy of Cymbeline. But there's nothing tragic about the ending, right? Is this play just called a tragedy because there's nothing else Shakespeare could have called it? (If you want to know more about this, check out what we have to say in our "Genre" section.)
So, why didn't Shakespeare give the play Imogen's name as the title? Well, usually his female characters are strongest in his comedies, but it would be hard to say that these comedies are "about" those female characters; they're more about groups of people and the issues they face. That could be why these kinds of plays get maxims (All's Well That Ends Well, Much Ado About Nothing, As You Like It) or plot hints (A Midsummer Night's Dream, The Taming of the Shrew, Love's Labour's Lost) as titles.
Or maybe Shakespeare was just like, "I don't know what to call this baby, so I'm just going to slap Cymbeline all over it and call it a day." It could have happened.