The Jews considered dead bodies unclean, and whoever touched them, even accidentally, had to engage in lengthy and tiresome rituals of cleansing. It was the duty of the Jews to whiten all the graves around Jerusalem with bright white lime that the pilgrims could avoid them because of the ceremonial pollution contracted by the touch of a grave.
Jesus explained to the hypocrites of His time that they, themselves, looked like these whitewashed tombs. From outside they looked clean and bright, but inside they were filled with death and corruption. Every impurity dwelled in them. They were aware of their sins, but behaved as if they were good. They continued in their wicked behavior and sin in spite of confessing the truth. Christ called the hypocrites bad-smelling tombs, and such a term was considered a serious insult at that time. With severity, Jesus tried to shake the elders of his people away from their self-righteousness that they might recognize the true condition of their hearts. Yet they did not repent because their faith in themselves blinded them to everything else. We should not judge them, but ourselves instead, then ask God to cleanse the impurities in our own hearts.