Do you have the owners manual? If you do it should be explained in their. However, does the light stay on while driving or does it go off after a short period of time?
This is a picture of the M3 dash cluster. The symbol you described looks like the one on the very right hand side on the bottom. Is that correct? If yes, then that symbol indicates a problem with the transmission or related component. Have the car scanned for Codes and see what comes up. The Code that is given will give you an idea of what is wrong.