The "ladder" is a symbol of the ascending stages of the history of the Righteous preaching the Will of God among the nations. 

The "ladder" also symbolizes the "ascent" of the "Angels", the Messengers of God, i.e. the Righteous to the Truth of the Word of God by the study of the Holy Scriptures and the "descent" to the consciousness of the peoples by the proclamation of God's Will expressed in His Commandments.