What are expressions?

What does Rachel mean ? Didnt get it.

Julie: Well, Ross and I were in grad school together.

Ross: But we haven't seen each other since then. Well I land in China, guess who's in charge of the dig.

Rachel: Julie! Julie, isn't that great? I mean, isn't that just kick-you-in-the-crotch, spit-on-your-neck fantastic?

Monica: (to Julie) It's an expression.

Thanks in advance