Plain English Meaning
SKC means the observer looked up and saw no clouds anywhere.
Full Definition
In a METAR, SKC is the sky condition code reported when no clouds are observed at any level. It is used by a human observer to indicate a completely clear sky.
Context Anchor
Derivation
SKC is simply a contraction of the words 'sky clear', shortened to fit the compact format of a METAR.
Why Pilots Care
Tells pilots immediately that visual conditions are excellent and no cloud ceilings will affect the flight.