Line width refers to the width of the traces or conductive paths on a printed circuit board that affects various aspects of PCB design and performance. The line width of a trace on a PCB is important for determining the characteristic impedance of the transmission lines. It influences the impedance along with the dielectric constant and thickness of the insulation layer. Any change in the direction or width of the trace can create an impedance mismatch, leading to issues such as insertion loss, far-end reflections, and voltage standing wave ratio (VSWR).