Plain English Meaning
It's a free ground-to-cockpit service that shows you nearby aircraft that radar can see but that aren't broadcasting their own position, so your traffic display gives you a more complete picture.
Full Definition
A free ADS-B service that uplinks radar-derived traffic information from FAA ground stations to properly equipped aircraft. TIS-B sends data on aircraft that are detected by ATC radar but are not transmitting ADS-B Out themselves, so that ADS-B In equipped aircraft can see this traffic on their cockpit display alongside ADS-B-equipped traffic.
Context Anchor
Seen on cockpit traffic displays when an aircraft has equipment that can receive FAA traffic broadcasts.