Leading line is a concept that uses the idea of lines to lead the viewers eye throughout the image. Lines don't need to be so obvious as an a physical line it can be a perceived line. A perceived line can be created by several objects in a row or aligned in the same direction or shape. For example 7 flower pots forming an 'S' shape in an image with a subject at the beginning middle and end. Leading isn't a necessity for all images but many times it can make any image better.
Note the strength of the beams (and their repetition), and how they push your eye up into the picture. Leading line can be used in many ways, this is a simple example.