Back in 1921, a man named Alfred Watkins pointed out the fact that, even very far back in history, people seemed to create roads in geographically convenient straight lines (as in between places of interest). People involved in occult groups picked up on the idea and assumed these must be magical lines our souls know about, and follow automatically. Ley lines are supposedly alignments between places of power, through which magic flows from one place to another. Where two or more intersect, there's a node or nexus, usually resulting in a place that's spooky or strange.
From a classical perspective on magic, ley lines are geographically straight, based on Watkins' observations, but there have been so many modern interpretations that I don't think it matters much.