Not sure if this is some sort of new trolling meme (like, what is air?) but this phrase has been popping up everywhere on Facebook, in many cases getting amongst the top comments.
It seems too popular to be a coincidence. My guess would be that it's from tumblr or twitter, if anywhere.
https://twitter.com/search?q=what%27s%20an%20ios&src=typd&f=realtime
Otherwise a large proportion of people are just genuinely confused about what iOS7 is, and we can all be on our way.