A Reference Source is usually a physical or digital document that you would refer to for more information about a topic. When doing academic research, it is sometimes much better to consult a reference resource than to type your question into google. Reference resources are usually curated around specific topics, making them more credible and informative than the advertisement suggestion that will pop up at the top of your google search!
Examples of Reference resources:
Selected books, journals articles, and reference books published by Cambridge University Press.
Encyclopaedia Britannica Public Library Edition.
Online encyclopedias, dictionaries and other reference books on all subjects, published by Oxford University Press.
Note that in CrossSearch, many topics, events, groups, etc., have a "Research Starter" entry at the top of the results. This is a great way to get started, often with timelines, jumping-off points for other references, and important terms and themes in that topic.