I was told my truck was running rich, what does this mean?

Hello, "Running rich" means that too much fuel is going into the engine compared to the spark and air needed to run the vehicle. This can happen for several reasons, but the two main areas of concern would be: 1) Fuel injector(s) stuck open. This means that more fuel than is being called for is entering the combustion chamber, causing a rich running condition. 2) The engine's computer is not seeing enough fuel mixture at the engine as it analyzes exhaust gases and air / fuel mixtures. This could be because of a vacuum leak at the engine, allowing far too much unchecked air into the engine -- meaning you're overwhelming the engine with air without knowing it, and the engine's computer tries to overcompensate by dumping more fuel into the combustion chamber to counteract the air. Testing needs to be performed to determine if it's a vacuum leak or mass air flow sensor concern (more likely than a fuel injector providing too much fuel concern).