2006 Chevrolet 1500 battery voltage drop

My battery voltage gauge is showing from 14.2 to 12 volts what is causing the drop in voltage.
Thank you

While the engine is running the voltage should be a steady 14 volts or so. Dropping to 12V while the engine is running is telling you that maybe the alternator is failing. If it only drops to 12V when you turn off the engine, that is normal.