What does D3, 2, and 1 mean on my car?

I drive 99 honda accord auto. My gears say D4, D3, 2, and 1. What do they mean and what are they for?

Its your gears for your transmission if you wish to manually up shift gears or limit the max gears it can go into.

These are alternate gear selections in the event you are traveling up steep hills you should never shift into these selections while moving forward only when sitting at a stop with your foot on the brake keep in mind when you accelerate your car is in a lower gear and will jump if you use too much gas pedal practice before you actually need these selections

Drive 4 is the highest gear, Drive 3 is a little slower than that, Drive 2 is the middle gear and Drive 1 is the low gear (granny gear).

These are positions where the transmission is inhibited from changing up. In D1 for example it remains in the equivalent of 1st gear on a manual (stick) gearbox, In D2 it is limited to the equivalents of first and second on a manual, though it can change up and down between them. D3 will restrict to 1st, second and third, and so on.

You can use these positions to "rein in" the transmission if you want to stick to the lower gears, perhaps for more control when descending steep hills or for maximum acceleration.

The Gear Selector will show P, R, N, D, 3, 2, 1
Normally you will be in P to start your car (It will also start in N)
In my Vauxhall I have to lift a coller under the Selector Knob to enable me to go from P to R to N after that not required as I go to D (As you say D, 3, 2, 1 this shows you have a 4 Speed Gear Box (D being top.
As you accelerate from idle (1,000 RPM) you will always start in 1st gear and change up every-time you hit 2,500 RPM. You will always go sequentially 1st, 2nd, 3rd and 4th up and down depending where you set the accelerator.
If you are going up a steep hill (Or towing a caravan) in D and the car keeps changing down and up you select 3 after it changes down. This is you telling the car not to go higher than 3rd gear. You can do the same in all gears lower ones in fields ect.
If you have a TC button this switches On/Off the Traction Control, Light comes on to show when Off
If you have a S button this selects Sport Mode and the car changes up at higher Revs (4,500 RPM)
If you have Winter or Snow Flake button Select before you set off in snow and it tells the car to start off in 2 and gear (D selected) Some will switch Off automatically when you get up to 3 rd but check hand book
If you have a button with the picture of a seat on it leave it alone and call money penny

Andy C