What do the markings of D2 & D1 signify in cars with automatic transmission?

Answer 1

D1 it stays in Low first gear as said above to slow your car with the gears going down hill or pulling a boat up a launch ramp.
D2 it stay in middle/second gear. This is good for slowing down hill and starting out in mud to keep the wheels from spinning, because you not have the power of First gear.

Answer 2

D1 and D2 are there to help give you more control when you don't want to car to automatically shift into higher gears- such as going down a mountain, or getting unstuck from a mudhole.

while the D- position will automatically shift all available gears ( 3 or 4 whichever you have)

D1 - will stay in first (the lowest gear) it will not automatically shift.
D2 - will automatically shift between first and second, but will not shift into third (or fourth if you have a 4 speed automatic)

Answer 3

D1=1st Gear
D2=2nd Gear