Skin Effect is caused due to eddy currents form due to the magnetic fields created when current flows through the conductor. These eddy currents are strongest near the centre. The magnetic fields oppose the flow of the current. Hence, the current finds it easier to flow across the periphery of the conductor.
Conductors which carry AC such as busbars in substations are made hollow for this reason as current flows only along the surface. The Skin effect becomes more pronounced at higher frequencies. That is why radio antennae are made hollow.
Conductors in overhead lines and in cables are generally made of strands instead of one solid conductor.
Skin Effect does not occur when conducting DC.