what does over drive on the shift in a car mean? What does it do when you press it??

http://en.wikipedia.org/wiki/Overdrive_(mechanics)

that button turns off the over drive [fourth gear]
the only time you usually use it is if you are pulling a trailer

It means to enable the overdrive gear in the transmission. The overdrive gear multiplies the speed of the speed of the engine, rather than reducing it. In other words, the engine runs slower in overdrive gear, which saves gas.

Overdrive is when a drive gear in the transmission is numerically lower than a 1:1 ratio such as 0.64. The button on your shifter usually turns off overdrive and tells the car to only go into the gears before the overdrive gears. Such as holding in 3rd gear in a 4spd car.

all overdrive is is the gear arrangements in the transmition. when u r driving on the highway or for long periods of time is is best to have the overdrive on. it will keep the transmition in a lower gear so the engine wont rev to high & blow up. my older brother blew out the tranny in our dads 'burbin driving to california because they had the O.D. off. it also gives u better gas mileage on the highway.

the only time u should have the O.D. off is when u r either racing or doing a LOT of stop & go traffic, like if u were in the city.