Please see The Carols We Love: The "Story Behind the Story of Twenty-two Classic Christmas Carols, Daniel Partner, Barbour Publishing, Uhrichsville, OH, 2002.
This song was first written in Latin in about 1200. It was sung in monasteries for vespers as part of advent. It has five verses, but each verse calling the Lord by a different name, imploring him to come. The names are: Emmanuel (God with us), our Wisdom from on high, Our Lord of might, Rod of Jesse's stem, Key of David, Dayspring from on high and Desire of nations. In using these different names, it points to different aspects of the Savior, who is God, who gives wisdom and shows they way, who is the descendent of Jesse and David, and holds the key to the kingdom of God, He chases the darkness away, and we yearn to come onto Him.