Plain English Meaning
It is the airspeed reading after the small built-in errors of the gauge and its plumbing have been taken out. It is a more honest version of what the airspeed indicator is showing you.
Full Definition
Calibrated airspeed (CAS) is indicated airspeed that has been corrected for installation error and instrument error in the airspeed indicating system. It is the speed the airspeed indicator would show if those small built-in errors were removed. KCAS expresses calibrated airspeed in knots. CAS still reflects the dynamic pressure the aircraft is flying through and is not corrected for air density (that further correction produces true airspeed).