[jcp123]

Sorry. PIDs = parameter IDs. Strictly speaking, this is any data read directly from any sensor on the vehicle by the computer. Examples would be throttle position, ignition timing, coolant temperature, and the like. I use the term more loosely to include anything a UG or SG would display, including numbers calculated from those raw sensor inputs like MPG, engine load, and the like.

ScanGauge on its own has 16 "things" or PIDs it can display (xgauge capability is completely different, based on advanced "mode 6" diagnostics, and will be ignored for now. It is best for ASE master technicians and advanced ecomodders). UG can support up to 77 different "things" or PIDs to display. More data = better, because even if you don't understand how to use it now, you can always learn at some point.

Fuel trim is basically your computer's compensation for real world conditions. It shows how much % of fuel it has to add or subtract from the computer's baseline, pie-in-the-sky, ideal conditions programming. As a budding DIY mechanic, it is a really good basic piece of information to use in diagnosing problems. Even pro technicians will usually use it as the first thing they look at after check engine light trouble codes to help diagnose a lot of running problems, especially where misfires are concerned.