Not sure what this is.

I just bought a 1994 Sentra. Did not have a owners manual. The other day I pulled the shifter into "drive" and I noticed a light on the dash came on. After trying to find out what caused this, I found a button on the side of the shifter lever (automatic). It turns something on and off, I'm guessing, but not sure what. A yellow light comes on from the left hand side of the dash.
Anyone know what the button is for?

Its prob overdrive.

overdrive. when the light is on it prevents an extra shift while cruising. the extra shift puts it into 4th gear and saves mileage. if overdrive is off then you stay in 3rd and the car has more response since the rpms stay in a slightly fatter part of the powerband and when you step on the go pedal it doesnt have to use that extra time to upshift.

Thanks, good to know.