The Emperor of the Eastern Roman empire, Justinian, looked at his empire and saw that the laws were a mess. Because they weren't written down, the laws in one part of the empire might be different then the laws in another part of the empire. Justinian wanted all of his people to be treated the same way, so Justinian had his judges and lawyers get together and write down all the laws of the land. They also wanted to write down the laws that began in ancient Rome, the laws called the Twelve Tables.
Once they had written down all the laws and made sure that laws did not conflict with each other, they gave this body of law a name. They called it the Justinian Code.
Many countries in the world have used the Justinian code as the basis for their own laws. The men who wrote the American bill of rights used the Justinian code as a basis or starting point to write the bill of rights.