Shin effect is that effect which does not allow equal distribution of current over the cross-section of the conductor in alternating current system. Since the total flux linking with the inner portion of the conductor is more than the outside portion of the conductor, the inductance of the inner portion is more than the outer portion of the conductor. Hence current flows with greater density at the outer surface of the conductor. Now this increase in current density in the outer layers results in a greater I2R loss for a given current and the effective resistance increases.
At low frequency the skin effect is very small, but at high frequency it is very large.