Fall line (wet phase), Mt Adams, upper Big Muddy valley, 2016.
(1) The fall line is the track that your pack follows when you get pooped, stop for a break, let it slide off your back onto the ground, and which it then tumbles along as you watch it head off on its own, downward, over the edge, ever downward, toward some hidden evil that sucks it deep into the brushy darkness where it is consumed by dirt monsters. Bye, pack. So sorry, but I'm not going there, 'K?
Fun fact: If you have a basketball with you, and drop it, or even just set it down, it will follow the same route as your pack. Why you'd be carrying a basketball on a backpacking trip is a great question, but one that no sane person can find a reasonable answer for, so think about it if you want, but on your own time, please.
(2) The fall line is the most direct route downhill from any point. This is why building a trail along the fall line is so great. It immediately shows you the effects of soil erosion since water just loves to run along the fall line, and as it does so, giggling madly, it rips out great swaths of your new, carefully-crafted trail. So more repair work for you then. Job security and all that as well. (See? Even the stupid can find a certain kind of tedious success in life.)
(3) Another kind of fall line, a sort of horizontal one, separates upland and coastal areas. The upland part may be rocky and generally lumpy, with the coastal bits being flat and goopy. This fall line can often be dramatic too, in its own way.
Say a stream crosses it. Fine.
If so, you'll usually see at least rapids, and maybe waterfalls, sometimes dramatic waterfalls. In the eastern U.S. the cities of Boston, MA, Pawtucket, RI, Troy, NY, Trenton, NJ, Washington, D.C., Fredericksburg, Richmond, and Petersburg, VA, Raleigh, NC, Columbia, SC, and Augusta, GA are all at the fall line, but on the flat, goopy side where it's easier to build cities.
On one side there be dragons, jersey devils, the wendigo, your sidehill gougers, scattered wild haggis, wandering wapaloosies, sliderock bolters, snoligosters, mosquitoes, and tourists, while on the other, goopier side, well, you have civilization (such as it is these days): parking lots, airports, traffic jams, smog, and random gunfire.
(4) The fall line is the steepest line across a given contour, which is the direction water flows down a slope if left unmolested, as noted above. This is sometimes called "the path of least resistance", usually defined as straight up or straight down a slope, though in the real world that "least resistance" thing is never up a slope, ever, in any way, shape, or form. (Figuring out why is left as an exercise for the reader.)
(5) A root that trips everyone in a line of hikers naturally forms a fall line — it's like a chorus line but uglier, with more swearing, and is composed wholly of the inept and stinky. Just as amusing though.
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Me? Still trying to remain upright.