Imagine being faced with a rocky incline so steep that you couldn’t possibly walk up it. What would you do? In all likelihood, you would get down on all fours and clamber up the precipice using your hands and knees.
Extending this analogy to a four-wheel-drive vehicle, they are already using all fours (tyres, in this case) to generate their forward propulsion, but when faced with extreme terrain, even this isn’t enough on occasions. Hence the provision in dedicated off-roaders of low-range gearing, which gives you a crawl ratio for when the going gets gnarly.
What on earth is this? Well, it’s basically a means of enabling the engine to spin faster than the axles, which means a lot of torque (twisting force) is transmitted to the wheels even at very low speeds. This in turn enables you to literally "crawl" over boulders and large obstacles, because trying to conquer this terrain using sheer momentum would be a sure-fire recipe to break your vehicle.