What Is ADS-B? Definition & Meaning in Aviation
ADS-B (Automatic Dependent Surveillance–Broadcast) is a system in which an aircraft works out its own position from satellite navigation and broadcasts it, together with altitude, speed and identity, about once per second.
In plain language
Anyone with a simple radio receiver on 1090 MHz can pick up these broadcasts. That is how flight tracking websites get their data: volunteers and organisations run receivers, and the signals are combined into a live map.
Example
When a flight from Karachi to Lahore is in range of a receiver, its ADS-B signal appears on this site as a plane symbol with a callsign, altitude and speed.
Why it matters
ADS-B is the reason public flight trackers exist. Because the signal is open, anyone can receive it, which makes live aircraft positions available without special access. Its limits, such as missing coverage at low altitude, explain most of the gaps you see on a map.