Plain English Meaning
A free service where ATC keeps an eye on your VFR flight and tells you about other aircraft nearby. You are still flying the airplane and looking out the window — they are just a helpful extra set of eyes on radar.
Full Definition
An optional service provided by air traffic control (ATC) to aircraft operating under visual flight rules (VFR), in which a controller monitors the flight on radar, provides traffic advisories about other aircraft, and may issue safety alerts. The pilot remains responsible for navigation, terrain clearance, and seeing and avoiding other traffic.