The Pharisees ask Jesus for a miraculous sign, presumably to prove his credentials. Jesus says, "OK, I will give one great sign for you. Just as Jonah was in the belly of the fish for three days and three nights, I will be in the earth for a similar amount of time."
Jesus is speaking about his death, that when he died, his body would be in the belly of the earth approximately three days and three nights. But he also implies that would be the TOTAL duration of time that his body would be dead. After three days, something extraordinary would happen, something that had never happened before and has never happened since. He would arise from the dead to live forever! That would be his sign to prove that everything he said was true.
And one GREAT sign it is. There is no greater miracle ever recorded in any religion, including the Christian religion, than the miracle of Jesus’ resurrection from the dead. This miracle verifies what Jesus said is all true--he is the Son of God who came to die and give his life in our place in order to save us from death and sin!