Plain English Meaning
Weather data sent up to the aircraft while flying, so the pilot can see current weather on a screen instead of only hearing it over the radio.
Full Definition
Weather information transmitted from a ground-based or satellite-based service provider to an aircraft in flight, displayed on a cockpit screen or portable device. Common uplinked products include radar imagery, METARs, TAFs, AIRMETs, SIGMETs, winds aloft, and lightning data, delivered through systems such as SiriusXM Aviation Weather (satellite) or FIS-B via ADS-B (ground-based).