Each section represents a world-ruling superpower. Each succeeding metal is less valuable, but each succeeding metal is stronger, as each empire was more powerful than the last.
This section represented the empire of Babylon, of which Nebuchadnezzar was king.
Silver—Chest and Arms
The silver chest with two arms signified the empire of the Medes and Persians, which conquered and supplanted Babylon.
Bronze—Belly and Thighs
This section represented the Greco-Macedonian Empire of Alexander the Great, which swallowed up Persia.
The two legs of iron represented the Roman Empire. After Alexander's death, his Hellenistic empire continued in a divided form until its divisions were taken over by Rome. The two legs apparently signified the east-west division that characterized the Late Roman Empire.
Iron & Clay—Feet and Toes
Extending from the legs are feet and toes of iron mixed with clay—a brittle and unstable mixture because it would not bond well. These represent the final phase of the Roman Empire, which will be made up of ten kings, some strong and some weak.