As TimM explained, the transferbox is what gives you hi and low ratio options. In the more modern Land Rovers, it also can give you centre differantial lock, while the older Series Land Rovers lack this as they do not have a centre differantial. Series Land Rovers use part time 4WD, and there only use 4WD when selected, thus there is no need for a central differantial, unlike the 90s and 110s and other more modern Land Rovers such as Disco 1s and some 2s, which have permament 4WD and therefore need a diff in the centre to avoid transmission wind up.
The transferbox is also what splits drive between the front and rear propshafts.