https://www.mdpi.com/applsci/applsci-11-08410/article_deploy/html/images/applsci-11-08410-g001.png

I found this good picture of it because I also had no idea what it means specifically. Like I know train tracks are dangerous, but IF the 3rd rail was fine to touch and you put me on the tracks and told me to touch the 3rd rail, I would have no idea what to touch.