Plain English Meaning
FIS-B is a free weather and flight information feed sent up from ground stations to aircraft that can receive it. It gives the pilot things like radar pictures, weather reports, forecasts, and airspace updates in the cockpit, at no charge.
Full Definition
A free data service transmitted on the 978 MHz UAT frequency that delivers weather and aeronautical information to suitably equipped aircraft. FIS-B products include graphical and textual weather (NEXRAD radar imagery, METARs, TAFs, AIRMETs, SIGMETs, , and ), , , and . FIS-B is one-way, ground-to- only, and is within of an ground while the aircraft is .