it's a device that you hook to your diagnostic port on your car. You follow the directions on the device. This device will read the codes your car's computer has stored. These are called DTC's or diagnostic trouble codes. By reading the codes and looking them up in the book, online, or on the reader itself depending on the model you can find the problem or get hints to the problem. A word of caution though. Just because you don't pull a code doesn't mean your car is running quite right.