Roads, fences, shorelines, staircases, shadows, and rows of windows all function as lines. The eye follows them almost involuntarily, which makes them one of the most reliable compositional tools available — you are steering attention using something already in the scene.
Direction changes meaning. Lines converging toward a distant point create depth and pull the viewer in. Diagonal lines feel dynamic and unstable. Horizontal lines feel calm and settled. Curves — an S-bend in a river or path — slow the eye down and make it wander rather than dart.
The failure mode is a line that leads nowhere, or worse, straight out of the frame past the subject. Before shooting, it is worth tracing where each strong line actually terminates. Lines that exit at a corner tend to feel intentional; lines that exit mid-edge tend to feel like an accident.