This was one I took mental note of awhile ago. I don't recall the circumstances. I think I know what it means in a general sense--"a plethora of" means an abundance of, or more than enough of--something like that. But what does it mean more precisely or originally?
You'll notice that no one ever exactly provides an answer to the question...
A plethora is more an excess of or overabundance of than sheer abundance, though the sense of simple abundance is now also in the popular vernacular. That's why it was once used to describe a blood condition characterized by an excess of red blood corpuscles. Or earlier an excess of bodily fluids. It comes, through Latin, from the Greek plethore or "fullness". The sense of excess apparently comes later, at around 1700.
This is a perhaps rare case where a word has drifted eventually back to something more like its original sense.