This is an exclamation used in the same sense as "well" "eh" and "hey." It could be used in the Hebrew/Yiddish translation of any of the following:
1. Well, do you want the egg roll or the knish?
2. Hey! Stop throwing paper airplanes in class.
3. My experimental tofu-liver-garlic cholent tastes good, eh?
4. So, Becca, I hear you and Izzy went out last week. Well?
5. A rebuke (on small kids): "Nu, nu, nu, you spilled all the milk!
6. To express doubt: "I heard that Rabin met Asad. Nu."
7. When the news ain't new no more: (the change is in the tone of the "nu").
8. As "come on": NU BEMET.
9. When one can't talk (i.e. in the middle of Shmone-Esre, after Netila before Hamotzi, etc.)