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

Answer 1

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.

Answer 2

5 ¶ So the people of bNineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.

6 For word came unto the king of Nineveh, and he arose from his throne, and he laid his robe from him, and covered him with sackcloth, and sat in ashes.

After Jonah had given his message to the people in the city they repented of their wrongdoing. To show this they put on sackcloth and covered themselves in ashed as a sign of having repented of their evil ways. Sack cloth was made of a course material.