Jesus said in the title verses above, "When an unclean spirit goes out of a man, it passes through waterless places seeking rest, and does not find it. Then it says, 'I will return to my house from which I came'; and when it comes, it finds it unoccupied, swept, and put in order. Then it goes and takes along with it seven other spirits more wicked than itself, and they go in and live there, and the last state of that man is worse than the first. That is the way it will also be with this evil generation."
To understand what He meant, we have to understand the entire context. The religious leaders said that Jesus was casting out demons by the power of Beelzebub the prince of demons; in essence Satan (Matthew 12:24). Jesus exposed the foolishness of their logic. He said, "If Satan casts out himself how can his kingdom stand" (Matthew 12:26-27)? In other words, why would anyone fight against themselves? Why would anyone tear themselves down? That is lunacy. The Pharisees' jealousy and hatred of Jesus blinded their reasoning to the degree that they would say anything to discredit Him; not realizing the enormity and danger of their hatred Jesus had to warn them that blaspheming the Holy Spirit would place a person in an unforgivable state with God! You never want to attribute the power of God to that of the devil, ever (Matthew 12:31-32).
Jesus also made it clear to them that they were against Him because of their adversarial attitude. They were not in support of Him or His ministry.
Jesus provides two Old Testaments accounts of those who were wise and listened to God's word: the Ninevites during the preaching of Jonah, and the Queen of the south who came from very far to listen to the wisdom of Solomon. Jesus said that both the people of Nineveh and the Queen of the south will rise up in the judgment and condemn His generation because they heeded the Word of God in their day and one greater than Jonah and Solomon was in their midst, namely Jesus Himself (Matthew 12:40-42).
Finally, Jesus likened His generation to a man who had a demon cast out of him. The demon is looking for an oasis but does not find any. The demon decides to return to the house (i.e. man) that he formerly possessed. The man is cured but apparently has not changed his ways. So the demon returns and brings seven more unclean spirits more wicked than itself and they dwell in that house. The last state of that man is worse than before. Jesus said that is the state of the generation that He lived in. That is also the state of any unrepentant person or generation.
You see Jesus is the light of the world (John 9:5). The light shines on all so that darkness is removed. But when one loves darkness rather than light because of their evil deeds, wickedness will prevail in that person's life (John 3:19). When one is exposed to the true light of Jesus' love and power and decides to reject it, you will be worse off than you were before the light of Jesus came to you. It is as if you were cured of evil by God's power but you remained recalcitrant. Sadly, for a person like that, because you did not put on Jesus Christ (Romans 13:14), you have left yourself wide open for Satan to take control. When the devil returns, he will make sure he will have a greater hold on that rebellious person's life so that they are worse off later than they were before they were exposed to the love and healing power of Jesus Christ.
Two passages all need to take heed to, "Behold NOW (emphasis added) is the acceptable time, behold now is the day of salvation" (2 Corinthians 6:2b).
"Today if you hear His voice, do not harden your hearts as when they provoked me" (Hebrews 3:15).