This is the 800th most frequent Chinese word.
莫
"莫" means "do not," "no one," or "cannot," and is also used as a surname.
In this context, '莫' is used as a negation, meaning 'do not' or 'no need to'.
莫愁前路无知己。
Do not worry that there are no friends who understand you ahead.
Here, '莫非' is a phrase meaning 'could it be that', usually indicating a rhetorical or doubtful question.
他问我莫非你不去参加了?
He asked me, could it be that you are not attending anymore?
In this example, '莫' advises abstention, interpreted as 'must not'.