It means the onboard computer sees a problem with many differant possiblities. Usally a emission standard issue. If you have a shop you deal with for service that would where I would recommend you take it first. If you don't know where to take it get recommendations from coworkers, or friends.
My 2005 CLK 500 check engine light came on yesterday. Could smell strong gas oder. Took it to dealer and they stated it was the fuel pump. Repair will cost $1200 plus. This car has cost me a lot of money. Just last week they told me the radio going out was a bad amplifier. Think twice before buying this model. It leaked oil when i drove it home the day i bought it new. Oil pan gasket pinched. I bought a lemon. My car is 9 years old and only 69k miles