Plain English Meaning
How the spinning columns of air left behind by a larger aircraft move, sink, and drift after they form -- which tells you where they will be and how long they will be a hazard.
Full Definition
The predictable way that wingtip vortices generated by an aircraft move and dissipate after they are produced. Vortices trail behind and below the generating aircraft, sink at roughly 200 to 500 feet per minute, level off about 500 to 900 feet below the flight path, and tend to drift laterally with the wind. They persist longest in light winds and calm conditions and are strongest when the generating aircraft is heavy, clean (flaps and gear up), and slow.