The story of Jonah being swallowed by a great fish is one of the most famous incidents in the Old Testament. It illustrates how far God was willing to go to ensure that his wayward prophet took the message of repentance to the people of Nineveh.
This large city was the capital of Assyria, a nation despised by the Israelites. Yet the Ninevites repented when they heard of God's impending judgment, unlike the Israelites who kept ignoring similar messages of judgment sent to them. Thus the book of Jonah is a challenge to the self-righteous and ethnocentric attitudes of the Israelites.