This may be seen on a METAR (aerodrome met report - based on actual observations - valid for 30 mins unless superceded) or a TAF (terminal aerodrome forecast - valid for up to 30 hours - based on a forecast).

As far as we are aware, the term "SKC" - meaning "Sky Clear" is more used in North America, and it means that there are no clouds present (or no clouds forecasted - if on the TAF), usually below a certain specified altitude, as clouds at high altitudes are not significant for pilots usually. It is most often written on METARs when a meteorologist has written the METAR themselves, rather than when an automatic observation has been taken, which is very common these days, and they tend use a different code such as NCD (no cloud detected).

In the UK and Europe, you'll usually find NCD (No cloud detected), NSC (No significant cloud) and CAVOK (Ceiling and Visibility OK) more commonly used, though each has their own technical definition.