Plain English Meaning
A VFR pilot flying through a TRSA who chooses to talk to ATC and accept their help with traffic and spacing. Participation is optional, but once you opt in, you stay in radio contact and follow the controller's directions.
Full Definition
A VFR aircraft that has voluntarily contacted ATC and accepted radar traffic advisories and sequencing services within a Terminal Radar Service Area (TRSA). The pilot maintains two-way radio communication with the controlling facility and follows ATC instructions for sequencing to the primary airport, while still operating under VFR rules.