Jorge Flefel : The Sign of Jonah - The only sign given to the Jews!

How shall we understand and apply the sign of Jonah?
Does this mean that Jesus actually was three days and three nights in the grave, or ... ?

'An evil and adulterous generation seeks after a sign; and there shall no sign be given but the sign of the prophet Jonah: For as Jonah was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth.'
Matt 12:39-40