Code means multiple or more than 2 missfires....check your work around the alternator and see if you can see anything near the ignition which involved the alternator change out.

Make sure that the alternator is working fine. Load test it. 

Hi. For future reference, OBD codes are super easy to find online. Just punch into a web search, or use this site: obd-codes.com

The computer will show code P0300 when the engine is misfiring. It is probably the most common code I see. This forum is full of it (good idea to try searching for "impala misfire").   Misfire means combustion is not happening the way it should. There are many causes for this because the engine needs these things for proper combustion:

- Just the right amount of fuel and air coming in

- Fuel being injected in a consistent manner

- A cylinder that seals tightly enough for the cylinder to compress this mixture

- A hot spark at just the right moment

- A clear path for air to come in the engine

- A clear path for exhaust to leave the engine.

The engine can misfire if it doesn't get any of those things. (and probably others)

So now question to you ... are you thinking about fixing this yourself, or are you ready to take it to a mechanic?  Leaving it alone will lead to bigger problems.