A four speed auto transmission as compared to a manual, it is
roughly like this
P - Neutral with brakes
R - Reverse is engaged
N - Neutral without brakes
D (with overdrive off - no lights on dashboard) - highest gear
achievable: 4th gear
D (with overdrive on) - Highest gear achievable: 3rd gear
D2 (or S) - Highest gear achievable: 2nd gear
D1 (or L) - Highest gear achievable: 1st gear
So in D1 or L, it is just like engaging 1st gear on your manual
car, engine brake strong, powerful, but not fast. Sames goes for
D2, D...
Notice I mention highest gear achievable, not D2 is 2nd gear.
That's the difference between auto and manual. That is to say that
in D2, the transmission can still select first gear if it think
that the car need the power to move along, eg like when you are
very slow or going up a very steep slope.
If you are able to understand the logic, you would know why usually
people leave their selection in D, when driving, because the
transmission will select the correct gear for you, say 1st gear at
10km/h and 4th at 80km/h.
More slightly experienced auto transmission will know how to select
the gear by putting the lever at D and control using their foot
(accelerator). The transmission will be able to detect the amount
of power demanded by the driver at the moment and select the
correct gear for the driver.