We bought our house 2 years ago and thought this was just an unused switch, it doesn't turn anything on that we can see. Then the other day my husband was putting up a ceiling fan in the kitchen and opened up the cover. It says for AC only. Does anyone know why this is there or what the function might be? It does not turn on the AC, we have a programmable thermostat that controls it. Thank you!
It's almost certainly not controlling your air conditioner. The "AC" as LQLarry states is for "Alternating Current", not "Air Conditioner". Most household wiring components are rated for 120VAC RMS, and not for 120VDC. RMS basically means "average"; the AC waveform is sometimes higher than 120V, sometimes lower, but it balances out to about 120V (power grid voltage varies by area; anywhere between 110 and 125VAC RMS is considered "normal"). If you put DC current at 120V through the switch, that's a continuous load at exactly 120V all the time, and that can cause the switch to overheat and fail.
Trying to find out what a switch does when it isn't obvious can be a pain and two thirds, I know. The best way I have found is to find the breaker for that switch (you can use a non-contact voltage tester to be SURE the switch is dead), cut the power to it, then use a tone generator with alligator clips to "broadcast" a signal through that wire that you can then trace with a reader.
First, head to your local hardware store and ask for a "circuit breaker finder", like this one: Klein Tools Digital Circuit Breaker Finder. You will need a model where the tone generator (the side that plugs into the outlet) also has alligator plugs, or you can pick up an adapter kit, which will have a variety of things the transmitter will plug into that allows it to be used for light bulb sockets, other types of receptacles, and bare wire.
A breaker finder, when used for its intended purpose, generally works best when you have some "endpoint" for a circuit like a receptacle or light socket. Since you don't know what the switch controls, I recommend you do not try to hook it directly to the wires of the switch with the power on to find the breaker. Instead, you should hold a non-contact voltage tester up to the switch, and flip breakers until the beeping stops (the old faithful clock-resetting method; make sure you turn off all sensitive electronics before trying this).
With the power off (test ALL switches and wires in the box to be sure), unscrew the switch plate and remove the switch. There should be two terminals screwing into the actual body of the switch, with one more on the metal frame (that's for a ground wire). If there are three terminals besides the grounding terminal, that's a three-way switch and there's a whole new set of possibilities; stop reading, tell us and we can help you further along those lines. We need to find out which one of these is the "supply" wire coming from the panel. If the wiring is to code you can sometimes tell just by looking:
If neither of those apply, with the switch off and hanging out of the panel, turn the breaker back on, and probe the two terminals with your voltage tester (being extremely careful not to touch any bare metal with any part of you). The terminal that lights up the tester is your panel wire.
Turn the breaker back off. Attach one of the tone generator's alligator clips to the terminal on the switch that DOESN'T have the panel wire. Now, look behind the switches, and you should see a bundle of white wires. These are the neutrals; one of them completes the circuit to your unknown device. There should be a wire nut on the end of this bundle; unscrew it and clip the other alligator clip to the bare metal.
Turn the tone generator on. With the switch off, your tone generator should now be sending a tone through the wires on the "load" side of the circuit only; something in your house should be giving off a tone when you point the receiver at it. Point the receiver (it must be close as well) at receptacles, other switches, light fixtures, fans, etc etc. until you hear the tone. Whatever that thing is (a receptacle, light fixture, etc), some part of it is, or was, controlled by that switch. My money is either a switched receptacle (allowing you to turn off anything plugged into that receptacle, like a lamp, with the switch), or a light fixture that used to have a ceiling fan or other multi-switch device, but has been replaced with something simpler. You can take the cover off the junction point to investigate further (make sure ALL power is off to the whatzit using your NCVT), but depending on what it is you should have a much better idea of what the switch is or used to be just by knowing where the wire goes.