Most likely the car has been wrecked and the air bag was deployed at some point. It is expensive to fix so most people dont repair it after an accident. If this has been your car for awhile and you know it hasnt been wrecked... then I would suggest taking it to a mechanic to get it looked at. Trying to fix something like that yourself could be painful if you messed it up.
A depleted airbag battery backup usually causes this sort of a problem. If the airbag module gets wet or corroded, it causes illumination of the airbag light fulltime. A faulty airbag clock spring might also be the cause. Have the car inspected by a mechanic for proper diagnosis and repair.