Plain English Meaning
A short message from a pilot describing the weather they are actually seeing and feeling at a given altitude and location. Other pilots and weather services use these reports to know what conditions are really like up there, not just what was predicted.
Full Definition
A PIREP is a report of actual weather conditions encountered in flight, transmitted by a pilot to a Flight Service Station or air traffic controller. It includes details such as location, time, altitude, sky conditions, visibility, temperature, wind, turbulence, icing, and any other significant weather. PIREPs are then distributed to other pilots, briefers, forecasters, and controllers to supplement and forecast information.