This is indicating that your ignition timing is out of spec. It should tell you somewhere in the engine bay, usually on the underside of the hood what the correct ignition timing should be. On my car timing can be slightly advanced or retarded by adjusting the distributor im not sure how its done on an impala. Depending how advanced the timing may be you may be able to easily correct it by adjusting your distributor yet if this was caused by a slipping timing belt your going to have to re-align your cams and crank pulley.
Who ever answered this question needs to pull his head out of his @$$ & open a book. He needs to NOT talk about something he knows NOTHING ABOUT. The PO011 DTC refers to vvs (CAM SHAFT TIMING) The 3500 has a timing chain NOT A BELT