An organized event to allow anyone with a motorcycle, a reasonable set of protective gear, and the price of admission to gain familiarity with a racetrack and learn to ride faster, more safely than on the street.
Trackdays often have ambulances standing by, are run by an organizer who has corner workers who have flags to control the event, and are typically organized in 20-minute sessions of three groupings of rider skill - novice/street, intermediate, and expert/racer.
Racers use trackdays for determining setup or practice, or to bring friends to the racetrack for the first time, in the hope that they become "hooked", and can then go racing with someone else to help share expenses.
Trackdays are addictive, and can be expensive, since for about 10% of the riders, it leads to racing. And Peter Egan has noted that "roadracing motorcycles makes Heroin addiction look like a vague wish for something salty".
"Dude, if you wanna get some skills on that machine before you wad yourself into a ball in the canyons, get your hide to a trackday."