The light means that the car knows exactly where to start in diagnosis. There are error codes stored in the computer. Many chain-type auto parts stores will read these codes for free. The next time the light is on, get to one, have them pull the codes, write down the exact code (e.g. "P1234") and post it.
If the light isn't on, some code readers will also read "history" codes. Some auto parts stores might have code readers with that kind of capability, but this often means you need to pay a shop with better equipment to read them. All you need to do is ask: "can your code reader pull history codes?"
In any case, there is no point in guessing. You need to know the code(s).
However, at the very least check your transmission fluid immediately - follow the procedure as outlined in the owner's manual. Its nothing like checking the oil.