The wheel size listed consists of several parts - the first is the Wheel Size, for example - 12x9. The second is the Bolt Pattern, for example 4/110. If two are listed, such as 4/110-4/115, then that wheel has a dual bolt pattern. The last part is the Offset, for example 6+3.
When reading the Offset, B = Bell and N = Nose. The bell shape creates maximum clearance inside the wheel for brake and steering parts. The nose shape has a smaller drop center for ease of tire mounting.