Reference resources are a great way for you to get a brief concept of a person, place, thing, or event. Using a reference resource can often tell you the main events of a person's life or the main parts of a battle. While your professor may not want you citing a general encyclopedia such as our Encyclopedia Britannica, some may allow you to cite more subject specific resources as described in the links below.
The Grove Encyclopedia of Medieval Art and Architecture by We have Encyclopedias in print and electronic formats. The first two below are examples of ones available electronically while the second two are only available in print.
Encyclopedia of Medieval Literature by 
The Encyclopedia of World History by 
The Oxford Handbook of Medieval Latin Literature by We have handbooks in print and electronic formats. The first two below are examples of ones available electronically while the second two are only available in print.
The Oxford Handbook of Women and Gender in Medieval Europe by