An automatic transmission is a type of gearbox that uses a complex system of gears, hydraulics, and sensors to automatically change the gear ratio between the engine and the wheels. The 2 and 1 on an automatic transmission refer to the two highest gears in the gearbox - first gear and second gear. These are also known as high gears or forward gears. The 2 and 1 designation means that first gear is twice as fast as second gear. This is because first gear has a higher numerical ratio than second gear. For example, if first gear has a ratio of 4:1 and second gear has a ratio of 2:1, then firstgear is twice as fast as secondgear.
The purpose of having high gears is to allow the engine to run at its optimal speed while providing enough power to the wheels to maintain forward momentum. In other words, high gears are used when speed is more important than torque (or rotational force). For this reason, high gears are typically only used when driving on highways or open roads where there are no obstacles in the way.
It's worth noting that some automatics transmissions have more than two forward gears. For example, many modern cars have six-speed or eight-speed transmissions. However, regardless of how many forward speeds a transmission has, they will always be labeled with a 2 and a 1 to indicate that firstgear is twice as fast as secondgear.