The Check Engine or SES light is one of the most common warnings to come on the dash of todays cars and trucks.
This indicates the Powertrain Control Module (PCM or ECM) is seeing a problem in the emissions control system. A simple scan tool is all that is needed to read the stored trouble codes that the computer has in its memory. These codes give an indication as to what system is having a problem. Many of the large national parts store chains will also do this for free.
If the light is flashing, you need to have the car or truck checked by a mechanic as soon as possible. This is because a flashing SES means that the engine is misfiring bad enough to cause catalytic converter damage. This can be very expensive to replace.