Congratulations on your purchase. The gear markings are:
D = Drive
S = Sport (similar to 3rd in a manual car)
B = Braking (basically this is low gear such as 2nd in a manual car)
You can move beween D/S/B while the car is moving. However going to B at high speed may cause gear damage. Just like in a manual transmission you shouldn't go to 2nd gear while doing, say, 100km/h. That's just asking for trouble.
The B range of the transmission is used while descending hilly roads. There is some amount of engine braking effect which reduces the amount of braking required by the foot brake.