Belshazzar became a ruler in Babylon after Nebuchadnezar died.  He worshipped gods of gold and silver, and did not honor the true God.  In fact, he used the holy goblets that had been taken from the temple in Jerusalem and used them for himself, his wives, and concubines.  So God wrote a message on the wall.  Daniel interpreted the writing and told the king that God would end Belshazzar's reign and kingdom.  The prophesy was fulfilled that night, and Babylon was defeated by the Persians.