The "service engine soon" light indicates that the vehicle's on-board diagnostic computer has detected a problem with one of the car's systems. The computer processes input on system operating parameters while the car is active and illuminates the light if they fall outside of acceptable ranges.
When the light is on, the computer contains a trouble code that states which system is malfunctioning, which can be read by a handheld reader that plugs into the dashboard. There are hundreds of possible codes, and they only provide the general location of the problem. More specific troubleshooting must be done on the area to determine the root cause.