Plain English Meaning
A free service that uses ground radar to spot aircraft that don't have ADS-B and then broadcasts their positions to your cockpit display, so you can still see them as traffic.
Full Definition
A free ground-based service, available within the service volume of an ADS-B ground station, that uplinks traffic information about non-ADS-B-equipped aircraft to properly equipped ADS-B In aircraft. Ground radar detects these aircraft and the ADS-B ground network rebroadcasts their position, altitude, and movement so that ADS-B In equipped pilots see them on a cockpit display alongside ADS-B-equipped traffic.
Context Anchor
You encounter TIS-B when using ADS-B In traffic information on a cockpit display or portable flight display.