The phrase "accelerator position" refers to how much you press down on the gas pedal in a vehicle. It determines how fast the vehicle will go. Full definition
Trouble Code P0121 can set if there are faults in the wiring or terminal connections to the throttle / accelerator position sensor. (yourmechanic.com)
The driver controls may include a steering angle sensor for measuring steering position, a brake pressure sensor or brake pedal switch for measuring brake pressure, and an accelerator including a sensor for measuring accelerator position. (greencarcongress.com)
(If the battery is running low, the alternator will engage, obviously, regardless of accelerator position) (caradvice.com.au)