The exchange happened during a Bryce Harper at-bat, having something to do with Harper's pinetar or something. Harper was making some comments of his own and at one point Ozzie was pointing a bat at Harper/Johnson. We'll find out the details later, but until then, let's play detective.
Guillen went and talked to the ump during the inning a few times as well as between innings. See if you can read his lips in this exchange while Harper was up to bat with Bryce or whoever he is looking at:
Was it "F**k you kid?" It was def F something
Ozzie Guillen's explanation via Mark Zuckerman "I was just telling him how cute he was"
Update: Nats Enquirer has some additional screengrabs