"Performative utterance" is just a way of saying that a person mananges to make something actually be so by saying it is so. The spoken words are the deed that have desired effect.

Claudius does this masterfully in his first scene: he makes everyone feel wonderful about his having married Gertrude in a very shady circumstance, and he does it by telling them how wonderful it all is, and how it's worked out so beautifully for everyone. You all said this was OK, and look: you were right! Most courtier types wouldn't stand up and say, "I did not say it was OK, and look: I was wrong!"