Plain English Meaning
FIS-B is a free signal sent up from the ground that puts weather reports, radar pictures, and flight notices onto a screen in the cockpit, as long as the aircraft has the right receiver.
Full Definition
A free, ground-based service that broadcasts aviation weather and aeronautical information to aircraft equipped with a 978 MHz Universal Access Transceiver (UAT) ADS-B In receiver. FIS-B transmits products such as METARs, TAFs, PIREPs, AIRMETs, SIGMETs, radar imagery, , , and and , allowing pilots to view and in the cockpit.