A Who was one of a group of objects, perhaps beings, which one could store in a Who-ville.
A phrase which used the expression, "whos in Who-ville", usually implied that the topic of the phrase was the smartest or most powerful. It is most likely a reference to the midi-chlorians.
Behind the scenesEdit
In Horton Hears a Who, the Whos are a race of microscopic people who live on a speck of dust. It is very likely that Yoda uses the term to refer to the midi-chlorians.