高高 is the reduplicated form of 高 (tall). In Chinese, reduplicating a monosyllabic adjective (AA pattern) adds vividness, intensifies the meaning, or softens the tone depending on context. As a predicate or attributive, it means 'very tall' or 'towering'; as an adverb, it modifies verbs to mean 'highly' or 'aloft' (e.g., 高高举起 'lift high', 高高在上 'stand aloof'). This form is common in descriptive and literary language.