system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what exactly does the o/d button on my automatic car do?

I get that it stands for overdrive but as I know about as much about cars as I know about quantum physics (that is to say, not a lot) what does that mean? Should I be using it? Should I not have been pushing the button while I drove down the interstate?
gpt
Your car has a transmission, which has different speeds.  That allows the wheels to spin at different speeds relative to the speed that your engine is speeding at.

If you watch your tachometer as you accelerate, you'll notice the engine speed increases, and then the needle drops back down and starts increasing again - this is because your engine speeds up until it reaches the point where the transmission shifts to a higher gear.

The o/d button selects whether or not overdrive is on.  Overdrive is your highest speed, where your wheels are spinning faster than your engine (hence over drive - the wheels are "over-driving" the engine).  It's good for high speed cruising - you get highway speed at lower RPM, which means better fuel economy and quieter engine noise.

You can disable overdrive, and that means that the car can't shift into the highest gear.  This is useful if you're trying to accelerate on a hill.  Higher RPM means more power, so in situations where you need more power, you want to turn off overdrive.

Generally though, you should keep overdrive on.