Most machines that convey us across land are built with gears. Both cars and bicycles, for example, have a number of metal gears that can be shifted as we increase our speed. Low gears are for going slowly or climbing slowly up hills. High gears are for when we have reached a faster speed and want to cruise along quickly for a while. This idiom, to "put it into high gear," is simply saying that someone is moving too slowly and they had better speed up. We seldom use it when someone is driving a vehicle, however. Instead, we often say this when someone is acting lazy, not doing their work, walking too slowly, or even still sleeping in bed! It's another way to say, "Hurry up, because there is something important to do."