A citation is when an author gives credit to an individual for a quote, picture, or some other type of information. Reliable sources include citations because it allows the reader to look into the sources the author used in order to create their book. If the person looks into the resources used and they are not from accredited or peer reviewed works, it will discredit the author's argument.
This is especially true in the field of history. The most reliable sources are ones that contain several citations and have been peer reviewed by other professionals within the same field.
