Plain English Meaning
A pilot with a basic certificate that allows flying small single-engine aircraft for fun, but only under certain limits — for example, not far from home, not at night, and not in busy airspace unless they get extra training.
Full Definition
A pilot certificated under 14 CFR Part 61, Subpart D, holding a recreational pilot certificate that permits flight for personal, non-business purposes in a limited category of single-engine aircraft, with restrictions on passengers carried, distance from the departure airport, airspace used, time of day, and cross-country flight unless additional training and endorsements are obtained.