The Jewish people believed that contact with a grave caused a person to become unclean (Numbers 19:16), so tombs were coated with a white lime solution to make them stand out to keep people from touching them accidently.
Jesus used this image to describe the Pharisees. On the outside they looked pious and religious, like the whitewashed tombs that dotted the countryside. But on the inside, they were full of decay and corruption.
MATTHEW 23:27 – Ye [Pharisees] are like unto whited sepulchres, which indeed appear beautiful outward, but are within full of dead men’s bones, and of all uncleanness.