The engine is the part of a car, truck, or SUVs drivetrain that gets all the attention, but the transmission is also important. It controls the gear of the engine, which is necessary for it to function properly at different speeds.
Most people don't pay much attention to their transmission because it is automatic and doesn't require any input from the driver. However, the transmission plays a role in how the car feels at higher speeds and when there is an incline or decline in the road. More advanced transmissions have more speeds, which translates into a smoother ride. Some cars have manual transmissions which require you to change the gears yourself with the clutch pedal, but these are becoming rare in the US outside of a few special cases like sports cars where the owners like to have direct control of the gears.