What is Skin Effect?

When direct current flows in the conductor,the current is uniformly distributed across the section of the conductor whereas flow of alternating current is non-uniform,with the outer filaments of the conductor carrying more current that the filaments closer to the centre. this results in a higher resistance(actual resistance) to alternating current that to direct current and commonly known Skin Effect.

The effect is more the frequency of supply and the size of conductor.

A conductor could be considered as composed of very thin filaments. The inner filaments carrying current gives rise to flux due to the current carrying outer filaments does not enclose the inner strand.