Your car is a veritable reservoir of fluids and goops, from gasoline to wiper fluid to motor oil, each of which performs a specific function throughout the vehicle. When it comes to your transmission, the most important fluid of your choice is transmission fluid, which keeps the various parts of the gearbox stable and lubricated during shifting, spinning, etc.
Although transmission fluid can consume time and heat, it does not burn like oil; there should be more in there at a time. So, if fluid levels are low, that’s a red flag. The most likely cause is a leak, perhaps from a gasket or seal on the cooling lines. If you’re driving your car and you see a small trail of red water behind you, it’s leaking water.
You can confirm a water leak by checking your dipstick. If the stick is short, the fluid will leave the path. It is also possible that there is enough water, but it has been too long and it has started to freeze. Fresh transmission fluid is red and has a slightly sweet odor, while used fluid is brown and has a slight odor.