When the car starts moving, the drive wheels require a lot of torque, while the engine develops a small amount of torque at low speeds. In this case, you turn on 1st gear, and thanks to its gear ratio, the torque increases, and the rotation of the wheels slows down compared to the engine speed.
When driving on the freeway in 5th gear, the opposite happens. Its gear ratio is such that the engine speed increases when torque is transmitted to the drive wheels. In this way, at an almost constant engine speed, the desired wheel speed is achieved.
However, the kinematic connection between the engine and the gearbox can also be interrupted - for example, when you start the engine, stand at a traffic light with the engine running, or want to change gear. On vehicles with a manual transmission, this task is performed by the clutch. It also enables a smooth start as it equalizes the speed of the crankshaft and the gearbox input shaft. On an automatic transmission, the clutch function is performed by a torque converter. The main advantages of an automatic transmission are as follows: the engine does not stall, there is no need to change gears, which makes driving easier.