圆圆 is the reduplicated form of the adjective 圆 (round). Reduplication of adjectives in Chinese (AA pattern) adds a vivid, affectionate, or descriptive quality, often implying "nice and round" or "pleasantly round". It is commonly used with 的 (e.g., 圆圆的脸 'a round face') or as a predicate (她的脸圆圆的 'her face is round'). This form is more informal and expressive than the single character 圆.