Plain English Meaning
A system where your aircraft constantly tells everyone nearby — controllers and other pilots — exactly where it is and how it is moving, using GPS. It does this on its own, without anyone asking. If your aircraft can also receive these messages from others, you can see nearby traffic and weather on a display in the cockpit.
Full Definition
A surveillance technology in which an aircraft automatically broadcasts its identification, position, altitude, velocity, and other data, derived primarily from onboard GPS, at regular intervals. Air traffic control and other suitably equipped aircraft receive these broadcasts directly, without the need for a radar interrogation. ADS-B Out refers to the aircraft transmitting this information; ADS-B In refers to the aircraft receiving ADS-B broadcasts from other aircraft and services such as traffic and weather.