In a newly built house, the second floor in the upstairs bedroom creaks, what does that mean?

Could be a number of things from the flooring not being laid down right to a humidity problem in that area caused from whatever is under it to a leak around the window. The lumber could have been wet while put down and has now dried out. Without someone actually looking at the entire set up, you can't get an accurate answer. Hope you have an honest builder or call another one for a second opinion.

Well first off, I'll say that you're not in danger of your floor caving in or whatever you may be worried about. Lumber expands and shrinks depending on the temp and humidity around it; that could be it. Also the expanding and shrinking could cause the nails to come loose a bit. That squeaking could be the sound of the wood rubbing against it's nail / screw. Do you have hardwood or carpet flooring in that particular room? If wood (hardwood) or OSB underflooring isn't secured properly it can move around against fasteners or other wood surfaces and produce that squeak you hear.

Whatever it is, tell the builder you want it fixed, fast. It's one of a dozen little niggling jobs that most house builders will run a mile from once they have sold you the house. You must be assertive, be a pain in the neck and phone him all hours of the day and night so it's not worth his while leaving you in the lurch. Having said that, the fact that he is coming is cheerful news, maybe he is one of the good ones - believe me, they are in the minority...