A transmission or axle ratio. It splits the engine torque coming from the transmission and delivers it to output shafts that connect to driveshafts leading to the front and rear axles. Most transfer cases designed for off-road use are able to mechanically lock the front and rear driveshafts together, so each end of the vehicle gets an equal amount of engine torque.
In most transfer cases low range is around a 2.5:1 ratio. The Rubicon's 4:1 allows you to go slower to crawl over things and climb steeper obstacles and descend steep hills slower. Most off-roading is done at pretty slow speeds and not flying through the air like Baja racers.