When you're learning about a new topic, sometimes you just need some basic background information. Reference works (encyclopedias, dictionaries, almanacs, atlases, etc.) provide facts and basic information about people, places, movements, events, particular pieces of art, or whatever it is you want to learn more about. Wikipedia is an example of a reference work.
There are specific reference works that focus on topics in Art History. Many of these sources are available online via the library's website, and some are in print in the library.
Reference works usually aren't suitable for citing in your paper, but they can give you a good place to start when you need to know foundational information about your topic.