I began casting to a spot behind a rock, still puzzled by what he meant. That was sixteen years ago, and a lot of trial and error has brought an understanding of the magic and basic biology in what the old fisherman said. Trout inhabit specific lies in the river based upon their size. It's sort of like being able to reach the peanuts without having to get up off the couch. Trout can simply laze around in the slower water waiting for dinner to arrive.
Current seams are high percentage lies, especially during a hatch.
They're also great places to try first if you're new to a stream. Look below the surface for structure that may create sheltered pockets, then explore them. The best piece of gear you can own is a good pair of polarized sunglasses. Trout will be sitting in the slower water watching for food. Depending on the hydrodynamics of the seam, insects will either spinout from the faster water into the slower water, or the trout will move out into the current to feed.