Rachel: Guess What?
This is a set phrase we use in English. In most cases, when you say this, your meaning is something like, “I have some news. Are you ready to hear it?”.
Literally, you’re asking the listen to guess the news. But in reality, of course, we don’t want to be doing that.
So, 99% of the time, the proper response should go like this:
Person 1: Guess what?
Person 2: What?
And then Person 1 would share their news.
Sometimes, as in the case of Ross in this scene, you could guess once or twice, if you really believe you know the correct answer.
But generally, it’s easier for everyone if you just say “What?”
Throughout the series, there are a few times when a joke is made about “guess what”, where the characters actually try to guess. Usually Phoebe. 🙂