# He will rule ... break them into pieces

This is a prophecy from the Old Testament about a king of Israel, but Jesus applies it here to those to whom he gives authority over the nations.

# He will rule them with an iron rod

Ruling harshly is spoken of as ruling with an iron rod. Alternate translation: "He will rule them harshly as if striking them with an iron stick" 

# like clay jars he will break them into pieces

"He will defeat his enemies completely as if breaking clay jars into pieces"