i have a chevy 3500 box van with a code of PO894 transmission comp. slip. what does this code mean

The Powertrain Control Module (PCM) is detecting Torque Converter Clutch (TCC) slippage when the Torque Converter Clutch should be engaged.

There are many things that would cause this code to be set. Was that the only code present?

thank you for your reply yes it was.... but there hasnt been any noticable "feeling" like the tranny slipping, smells etc. other than the engine light coming on will we notice the ttc slipping? will it have any effect on driving.

It can negatively affect fuel mileage. The TCC in only engaged at higher speeds.