再三 means 'repeatedly' or 'again and again', emphasizing persistent, deliberate repetition—often with a sense of urgency, concern, or insistence. Literally, 再 means 'again' (used for the second time), and 三 means 'three', but together they form an idiomatic expression indicating multiple occurrences beyond just two or three times; it conveys intensity and continuity rather than exact counting.

This adverb typically modifies verbs and appears before them in sentences, often in formal or serious contexts: giving warnings, offering advice, making requests, or expressing worry. It frequently appears in written Chinese and spoken language when someone is urging caution or stressing importance—e.g., a teacher reminding students, a doctor advising a patient, or a parent warning a child. While not colloquial slang, it’s widely understood and used across age groups and registers.