Ever use Wikipedia? An encyclopedia or dictionary? Then you've used a reference source!
A reference source is essentially a resource designed to give you a quick, usually authoratative overview of a topic. It's not a book, for instance, to be read cover to cover - only to be read at point of need.
It's a great place to start when doing research as typically they will give you not only an overview of a topic, but clue you in to important people, names and places that can be later used as search terms. Additionally, many reference sources include citations or recommendations for further, in-depth reading on a topic.