There are 2 dipsticks under your hood: one to check your motor oil and one to check on that underrated worker bee, the transmission fluid. Transmission fluid performs many of the same functions as motor oil, like lubrication and cooling. As transmission fluid moves through an automatic transmission, it gets pressurized, which provides the hydraulic power needed to perform the transmission’s basic functions, like shifting gears.
For those of you with a stick shift, manual transmissions also require lubricant, but often use a different kind that has to be serviced from underneath the car (which means no dipstick).