In the library, we use the term "reference materials" to refer to things like dictionaries, encyclopedias, handbooks, or atlases. Using reference materials at the beginning of your research is a great way to help you clarify any terms you are unclear on and to find additional resources related to your topic. The better you understand your topic, the more successful your search will be.
Since these reference materials are edited and written by experts, they are more reliable than sources you may find from general websites. They're like Wikipedia, but better!