Answer: Jesus teaches us that all of these things that come from inside a person--from out of the heart/mind--and then come out of a person by what is seen (or sometimes actions which are not seen) makes a person "unclean," (verses 20 and 23).
MORE INFORMATION AND/OR OTHER SCRIPTURE REFERENCES:
Men look on the outward appearance. God looks at the heart and mind of each person, and nothing is hidden from Him.
1 Samuel 16:6-7: "When they arrived, Samuel saw Eliab and thought, ‘Surely the Lord's anointed stands here before the Lord.' But the Lord said to Samuel, ‘Do not consider his appearance or his height, for I have rejected him. The Lord does not look at the things man looks at. Man looks at the outward appearance, but the Lord looks at the heart.'"
Isaiah 29:15-16: "Woe to those who go to great depths to hide their plans from the Lord, who do their work in darkness and think, ‘Who sees us? Who will know?' You turn things upside down, as if the Potter were thought to be like the clay! Shall what is formed say to Him Who formed it, ‘He did not make me'? Can the pot say of the Potter, ‘He knows nothing'?"
Jeremiah 17:5: "This is what the Lord says: ‘Cursed is the one who trusts in man, who depends on flesh for his strength and whose heart turns away from the Lord."
Matthew 15:16-20: "‘Are you still so dull?' Jesus asked them. ‘Don't you see that whatever enters the mouth goes into the stomach and then out of the body? But the things that come out of the mouth come from the heart, and these make a man "unclean." For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false testimony, slander. These are what make a man "unclean"; but eating with unwashed hands does not make him "unclean."'"
