What does Jonah chapter 3 and verses 5-6 mean?

Jon 3:5 The people of Nineveh believed God's message. So they decided that everyone should fast, and all the people, from the greatest to the least, put on sackcloth to show that they had repented.

Jon 3:6 When the king of Nineveh heard about it, he got up from his throne, took off his robe, put on sackcloth, and sat down in ashes. - GNB

It means the people of Nineveh when they heard that God was going to punish them repented, they showed this by fasting, wearing clothes made of sack cloth and they put ashes on their heads.