Plain English Meaning
A heads-up from the ground network letting the pilot know if they are getting the full traffic picture from TIS-B right now, or if traffic service has just started, just stopped, or is unavailable.
Full Definition
A Traffic Information Service-Broadcast (TIS-B) message that tells the pilot whether TIS-B traffic uplink service is currently available to their aircraft from the ADS-B ground network. It indicates when the aircraft has entered or left the coverage area of a TIS-B service volume and whether traffic data being received is complete or not.
Context Anchor
Seen on ADS-B In traffic displays or avionics pages when checking whether broadcast is available.