I'm guessing the context here is a car or other motor vehicle.
Modern cars have a lot of sensors throughout the engine and associated systems. These are designed to detect faults with the engine, and to feed information to the engine management computer to allow the engine to adjust itself to operate at peak efficiency in all conditions.
If a fault condition is detected, a fault code is stored in the memory of the engine management computer, and this is generally indicated by the "check engine" light being illuminated. These codes can be read, or "pulled" by a mechanic, being a person who works on cars. The codes are sometimes very helpful in identifying what is wrong with the car.
Actually, the codes for most cars can be read using a smart phone and a cheap OBD2 Bluetooth adapter, but for information on that sort of thing you'll be better off on the motor vehicle stack exchange.