For each concentration, you have to take a certain amount of classes to fulfill a credit requirement with the concentration. Once you have done that, you have officially completed it (it's usually 3 classes).
This is on top of the "required" list of classes you have to take, which includes 3 foundations and at least one class in 6 out of the 7 "core" categories.
You will hear this often for not just Booth, but most MBAs: the concentrations don't matter and I don't recommend restricting yourself for certain classes to get a concentration.