Read Luke 11:30 Using Other Translations
For as Jonas was a sign unto the Ninevites, so shall also the Son of man be to this generation.
For as Jonah became a sign to the people of Nineveh, so will the Son of Man be to this generation.
What happened to him was a sign to the people of Nineveh that God had sent him. What happens to the Son of Man will be a sign to these people that he was sent by God.