Leading lines are pretty much the same as diagonal lines, it's just that these lines are straight. They are lines that are composed in a way to naturally make it look like as though it is pointing somewhere in an image. These types of lines are used a crazy amount of times in movies, so many that it just feels normal to see an image with this. Leading lines are used to 'point' at a significant part of the image in order to make that point stand out.
As you can see here, the grey spaces between the flowers are used to point at the windmill. Those lines at the bottom are what we call leading lines, they are really easy to find and use, but you just have to get the right angle for them. Leading Lines can really make your pictures stand out, but i personally believe that having leading lines do not really make anything have a deep meaning, they just make certain objects stand out.