The Holy Roman Empire is a group of nations that appears in most Pre-1800s Europe Scenarios. It functions with one nation (Pre-Selected) as the Emperor. It also consists of two electors, who are the two most powerful nations. The electors choose who becomes the Emperor. (Tiebreaker not known) It is easier to ally with other members of the Empire but has a negative deduction if you are making allies with someone outside of the HRE. Being the Holy Roman Emperor won't give anything to your civilization.