You establish a "pace count" for yourself on flat ground, ie, how many times your left leg hits the ground in 100m or 100yds. Then, if you're smart, you do the same thing on a couple of grades of uphill and downhill, and on long wet grass vs dirt vs gravel, and finally do all of those over with a pack on at your normal weight. Scribble them down or note it in your phone.
Now, when you're /out/ and going across ground where there are no paths, if you know for example your pace count on the terrain you're in is 65, you count every time your left foot hits the ground and when you get to 65, you shift one of the small beads up the cord. When you've gotten to nine of those and hit 100m/yds again, you shift one of the large ones and reset the small ones. Now you can reliably measure your distance across terrain really quite accurately without fucking up the count. By keeping a decent line with a compass and intermediate waypoints or fixes, you can walk through woods in a fairly direct line without GPS and maintain a pretty reasonably accurate idea of where you are the whole time.
Not super useful on premade trails though still handy to some degree; not super useful if your phone works and stays working, but it's a good old-school skill to have.