Two important things to bear in mind are that the eye follows a line, and that it also tries to construct a line from appropriate suggestions as a clear line provides a natural path for the eye, which moves along it.
Perhaps this is not the most elegant sentence Freeman has ever written, but the meaning seems clear. The mind constructs meaning through pattern and uniformity, which it seeks and even builds when it can't be clearly found. As producers of images, photographers can help viewers see by being aware of lines - explicit or implied - when composing.
