A direct current (DC) flows through a wire using all of its cross-sectional area. If you put alternating cuurent (AC) through a wire then, as the frequency increases, it uses less and less of the cross-section moving closer to the surface. At high frequencies it almost on the surface only of the wire and that is why sometimes silver plating is used as this is where the AC is travelling. This is known as the skin effect and is especially apparent in the microwave region and higher.
A waveguide turns this inside out and traps the radio waves inside a tube. You will often find that inside surface is silver plated and, as it is protected from waether, will last longer.