You already know what reference sources are.  You have been using dictionaries and encyclopedias for many years.  You might not know that an academic library will have lots of specialized encyclopedias, written by experts, that are very useful in the exploration phase of your research process.  Your library has many of these specialized tools, both in print and online.  These are useful because:

• They are written from a neutral point of view, intended to inform you about many perspectives on your topic.
• They are written by experts, for readers who are educated, but who are not experts in the field themselves.
• They point you to additional resources and related topics.