The behavior of AC current is different than the behavior of DC current. When DC current flows through a conductor, it get uniformly distributed over the whole cross-section of the conductor but when AC current flows, it has a tendency to flow near the surface of the conductor. This behavior of AC current is popularly known as Skin Effect.
For understanding the reason of Skin Effect, carefully observe the figure below.
In the figure above, I is the AC current in flowing in the conductor. This current I create a circular magnetic field H shown in blue as per the Right Hand Thumb Rule. The induces magnetic field in turn finds a way to discharge the stored energy in form of current Iw. In other words, the magnetic field induces a current in the circular loop of the conductor. This backward current resists original flow of current due to which the flow of AC current reduces reasonably. This effect is known as Skin Effect.
Now, there are some interesting points to be noted.
This is because the path of Iw current as shown in figure above throughout the circumference of H magnetic field converges at middle, therefore it will resists the flow of current through the centre of conductor the most.
Another thing is the effect of frequency on the Skin Effect. Skin effect increases as the frequency of AC supply is increased. Why?
Since, at low frequency the magnetic field gets enough time to discharge when current strength decreases / direction changes, the skin effect is not observed as the opposing force is distributed over long interval.
But at High Frequency, the time for stored energy to discharge is reduced; hence the energy builds up to create a sufficiently observable opposing force in form of Iwcurrent. That is why at high frequency the Skin Effect is more.