whenever I explain it to folks, I figure its not necessary to know the details of the term, but just to think of it as a black and white number that shows you a number to compare to diff setups, like diff cassettes and chainrings, or to compare bikes.
The thing is to have a reference to understand.

example, so back when I bought that Kuwahara, I set off on a fully loaded trip for the first time, four panniers, tent on top of rear rack, handlebar bag. Had too much crap, hadnt trained with all the said crap on bike, and I went somewhere with bugger hills that were a total killer. Had to walk sometimes which was a drag, and ended up with a wonky knee and had to take days off cuz I couldnt ride cuz of buggered knee, get to a local doctor to get anti inflammatories, biking to said office with one leg.....bad experience basically.
Got back and asked my friendly bike shop guys what could I do to lower the gearing on my bike, as it wasnt enough , as was abundantly clear to me.
They suggested changing out the small front chainring, or granny gear as we call it, from the stock 28, to a 24t. Was an easy peasy fix, and someone showed me a chart where I could get the exact "gear inch" number for my bike, pre and post change. Went from a low of 25 g.i. to about 21, and it made all the difference in going up steep hills.
So even if I didnt know the details about g.i, who cares, I now had a reference for my bike with X amount of crap on it going up Y type hills.

Go ahead through the years, and now I know that for certain types of touring, I want and need about a 20g.i. low.
From simple observing, I knew that for other types of touring that I was dreaming of doing, ie Latin America, where I knew the gradients are steeper than in N America and inFrance where I'd been on other trips, plus knowing that I'd be carrying more stuff, that I would want lower than the 19 g.i. on one of my bikes that I did ride in Latin America on a moderately loaded trip.
Hence me getting a bike set up with a 16.7 g.i. low, and as expected, it worked perfectly ---- so bottom line, having a reference and the charts that easily calculate your gearing, you can get a number that you can relate to your real world experience.

voila

as mentioned, the sheldon brown gear calculator is a good one,
and the "html gear calculator" shown my cycco above is also a great one.
with both, you put in the right info, check off "gear inches" and you get a value for your bike.

its very handy for tourers