A parking pawl is a device fitted to a car's automatic transmission that locks up the transmission. It is engaged when the shift selector is placed in the Park position, which is always the first position (topmost on a column shift, frontmost on a floor shift) in all cars sold in the United States since 1965 (when the order was standardised by the SAE) and in most other vehicles worldwide.
The parking pawl locks the transmission's output shaft to the transmission casing by engaging a pawl (a pin) that engages in a notched wheel on the shaft, stopping it (and thus the driven wheels) from turning.
Most manufacturers and mechanics do not recommend using the transmission's parking pawl as the sole means of securing a parked car, instead recommending it should be used as a backup for the car's parking brake. Constant use of the parking pawl only, especially when parking on an incline, means that driveline components are kept constantly under stress, and can cause wear and eventual failure of the parking pawl. Replacement can be an expensive operation since it generally requires removing the transmission from the car.
To keep the tension off the pawl, it's a good idea to first apply the parking brake and let the car "rest" on it before putting the car in Park position.