清纯 describes a person (typically a young woman) or their appearance, smile, eyes, etc., that appear pure, fresh, guileless, and untouched by worldly influences. It is more about an outward impression of innocence and simplicity than about moral purity; the English 'pure' is close but a bit too broad — 纯净 (pure and clean) is used for air and water, and 纯洁 (pure and chaste) for intentions or moral innocence. Use 清纯 when you want to say someone looks naturally sweet and unspoiled.