A line is one of the basic elements of art. The most obvious is a directly visible solid path. But, consideration of implied lines is more interesting. On a computer, a line is no more than a series of pixels or dots. If the pixels are close enough to each other, it is a solid line. When the pixels are in proximity of each other, there is an implied line, such as the image below.
Composition generates edges and numerous implied lines are formed. In the image below, 4 triangles are drawn next to each other. Their proximity and composition create the implied white lines.
Many straight lines together may even create the impression of a curved line. If you are familiar with the arts and crafts practice of tying strings to pins, then, the image below is a digital version.