The skin effect losses are a form of resistive trace loss. These losses are a manifestation of the basic principle that any conductor, upon the passage of current, heats up. Since part of your signal power is converted to heat, there is that much less power available at the end of a long trace to activate the receiver.

The exact amount of resistive heating depends on the distribution of current density within the cross-section of the trace.

What's important when considering the skin-effect is to remember that the interior of a trace is self-shielded by the top and bottom surfaces of the conductor.

I show in this figure a photomicrograph of the cross-section of a typical pcb trace. On the right I've marked the effective thickness of shielding (skin depth) required to provide a significant degree of self-shielding effect.

No magnetic fields, and thus no currents, penetrate to the interior of the conductor - therefore all the high-frequency signal current rides in a shallow band just underneath the surface of the conductor.

The self-shielding effect becomes progressively more pronounced at higher frequencies, further restricting the useful current-carrying cross-sectional area of the trace.