A B- curve means that the mean grade of the students will end up a B- (2.7). That's all the information it really gives. A student in a B- curve will overall have a worse GPA than someone on a B curve, but a better GPA than somoene on a C+ curve. The lower the GPA point they define as the curve, the harder it tends to be to get A's.

Note that to better define the curve, you also need to know the standard deviation and flatness. Some B- curves may give out only one A in 100, while others may allow six. It all depends on how many Cs, Ds, and Fs the professor/school is willing to hand out.