In the photo above, you have a classic example of the photographic compositional element known as “leading lines.” The banks of the creek as well as the trees on each side form lines that lead your eye to the edge of the visible creek. In this photo the viewer’s eyes are led from the bottom/middle of the photo to near the top.
Leading lines can perform the simple function in the photo above, in essence becoming the subject of the photo, or part of the subject itself. But leading lines can serve to anchor your main subject in the frame, like in the photo below:
In the above photo, and starting from the bottom left of the frame, the viewer’s eye is led by the long shadow to the main subject. But there is also a secondary leading line that is the lighter colored pavement. And last, the starburst (sun rays) can also be considered leading lines that serve to anchor the subject from the top of the frame. This is definitely not a conventional (whatever that means) leading lines photograph, but it still illustrates the use of lines and flow in the composition, leading the viewer’s eyes to the main subject. But as I said, leading lines don’t always need to anchor a subject, the line or lines can be the subject, as is the case here:
The trail is both the leading lines element and the main subject of the above photo.
And here is another example of leading lines that anchor the main subject, although in this photo, the leading lines are likely to lead the viewer’s eyes away from the subject (depending on where your eyes first hit the photo), but in this case it makes for an effective and interesting composition: