Plain English Meaning
Something has gone wrong with the small openings on the outside of the aircraft that feed air pressure to the airspeed, altitude, and climb/descent instruments. When that happens, those instruments stop showing accurate values and may be misleading or stuck.
Full Definition
A malfunction of the aircraft's pitot/static system in which blockage, leakage, or icing of the pitot tube, static ports, or associated plumbing causes the airspeed indicator, altimeter, and vertical speed indicator to display incorrect or unreliable readings.