What exactly does a "dimmer switch" do?

What does a "dimmer switch" do on a '93 Ford T-Bird? does it have anything to do with my headlights? or is it just for my hight beams? thx

The dimmer switch switches your headlamps to high beam when pushed forward and when in the middle postion the low beams are on. If you pull the switch toward you (flash to pass feature) both beams are on until you release the spring loaded switch and it returns to the middle position.

I thought it was to dim the interior lights only.

?

We may be talking about 2 different things. I grew up with a dimmer switch as what I described because you "dim" your headlights as you approach traffic.