莫 is a literary adverb meaning 'do not' — it expresses prohibition or a strong negative imperative, similar to 不要 but more formal and common in written or classical-style Chinese. It also appears in fixed classical patterns like 莫不 ('there is none who does not...'), though that use is more of an indefinite pronoun. As a surname, 莫 is common, but in this entry it functions as an adverb. Use it to tell someone not to do something in a concise, elevated register.