Plain English Meaning
The small computer that turns outside air pressure and temperature into your airspeed and altitude readings has stopped working properly. The numbers on your speed and altitude displays can no longer be trusted, and any system that relies on those numbers may also stop working.
Full Definition
A malfunction or loss of the Air Data Computer, the unit that processes pitot-static inputs (ram air pressure and static pressure) along with temperature data to calculate airspeed, altitude, vertical speed, and Mach number for the aircraft's displays and flight systems. When the ADC fails, the may show invalid or flagged airspeed, altitude, and , and dependent such as , , and may be degraded or unavailable.