An automatic transmission is a hydraulic system that operates on pressurized oil (fluid). This fluid lubricates the transmission and applies various friction devices (clutches and bands) at specific times to change gears and transfer power from the engine to the drive wheels. When the fluid drops below a safe level ,lubrication and hydraulic pressure decrease, causing excessive wear to major transmission components and in some cases, immediate failure.
With the self-service gas stations, drivers are not checking fluid levels of many major vehicle systems, as they should. These include motor oil, brake and power steering fluid, antifreeze, windshield washer fluid and of course, transmission fluid. Learn to spot transmission leaks. If you normally park in a garage or driveway, look for fresh (still wet) oil stains under the front center of the vehicle. If you park on the street or in different spaces everyday check for oil stains before and after you've parked there to see if you left any new ones.
Check the fluid about every 500 miles or every-other time you get gasoline. If it is low add the proper type of fluid until it reaches the full mark. Do not overfill. Although overfilling will probably not do damage because the transmission will push out any excess through its venting system, that excess oil could drip onto hot engine or exhaust components possibly causing a fire.
Transmission fluid does not evaporate, nor does it burn like motor oil might through an engine. If transmission fluid is low it has definitely leaked out somewhere. The best course of action is to bring the vehicle to Affordable Transmission & Performance immediately so they can perform a Free inspection and possibly save you a lot of money by catching a small problem before it becomes a major failure. If the level is low and you can see fluid dripping under the vehicle, do not drive it any further. Doing so could very likely cause additional damage. They will make arrangements to have your vehicle towed in, to avoid any further damage.