"Skin Effect" describes the tendency of an AC current to travel toward the outside of a conductor as frequency increases. Using a round, solid-core or stranded-bunch wire as an example, the current density would appear as if the wire itself were a hollow tube - migrating toward the outside of the wire as frequency increases and leaving the inside of the wire unused. Since the inside, or core of the round wire isn't used for higher frequency AC current flow, it becomes in our best interest to eliminate as much of the unused part as possible.
Litz (litzendraht) wire was an early solution to solving the current density issues for high frequencies transmitting in round wires. This approach bundles or bunches a plurality of fine wires which have been individually insulated from one another by a thin lacquer or enamel. The fact that they are insulated from one another ensures that they don't mimic one massive solid-core wire (which would otherwise suffer the ill-consequences of skin-effect because the individual wires are electrically continuous with each other along their entire lengths). Several brands of cable have used this approach to excellent effect, and this is certainly preferable to the ordinary (non-enameled) stranded cables on the market.
Another effort at solving the "skin effect" conundrum has been the Ribbon Wire (example) - a length of round wire that has been crushed between a capstan and a pinch roller to form a thin, flat ribbon. This presumably ensures an even density to current flow in the wire up to the skin-depth equivalent of the ribbon's thickness, but in practice this is affected by the position of the positive conductor to the negative conductor and results in variations of Proximity Effect forcing the current to migrate, or favor, one side of the conductor over another, and utilizing less of the conductor as frequency increases (therefore increasing the apparent resistance incrementally as frequency increases).
Over the years, we have always sought our own solutions to the two-sided problem of Skin Effect and Proximity Effect, and to that end have employed combinations of single-solid core wires at small gauges, we have emulated tubes by helically winding hundreds of tiny-gauge wires (38awg) around the surface of an insulator-core, and we have produced thin-walled pure copper tubes that have wall thicknesses of 0.008" (a skin-depth equivalent to 32 awg). In all, since AC current-flow in a round wire forces the wire to emulate a tube, we simply create a tube and force all current, from DC to the highest frequency, into it.
"Skin Depth" is defined as the depth below the surface of the wire where current density has decreased to 1/e or 37% of its value at the surface of the wire.
A important ratio is the ratio of AC resistance to the DC resistance of the conductor. Resistance is inversely proportional to the cross sectional area of the conductor.
Generally speaking, when selecting the proper wire gauge, one would prefer that the DC resistance be equal to the AC resistance at the highest frequency being propagated (ACR @ f). Simply put: the ACR @ f is equal to the DC resistance when the ratio is 1:1.
When this ratio is achieved, the wire is being appropriately saturated with current throughout it's cross-section from DC up to the specified frequency. In other words, the conductor has been designed so that there is no wasted inner core where AC current at higher frequencies doesn't tend to flow.