The transmission has a bunch of gears (normally 5+ forward and 1 reverse), and these gears all have different gear ratios. The purpose of these is so the car can move at various different speeds while still being able to keep the engine within its RPM specifications. For automatic transmissions, it’s more complex than just having 5+ different gears that you switch between, but each separate gear ratio it can produce is traditionally called a gear.
Along with preventing the car from revving too high and damaging itself, it also allows you to keep the RPMs low when traveling far distances or efficiency, or high when rapidly accelerating and/or driving fast for fun.
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