Plain English Meaning
The little pressure-sensing tubes and ports on the outside of the aircraft that feed three of your flight instruments have stopped working properly. As a result, your airspeed, altitude, or rate of climb readings can no longer be trusted.
Full Definition
A malfunction of the aircraft's pitot tube, static ports, or the plumbing connecting them to the flight instruments, causing the airspeed indicator, altimeter, and vertical speed indicator to display inaccurate or unreliable readings. The failure may be partial or complete and is most commonly caused by blockage from ice, water, insects, debris, or by a leak in the system.