When you use a quote, idea, or information from a source in your paper, you must cite the source in your Works Cited list. This is important not only to give credit to the original source (and avoid plagiarism!), but also to ensure that anyone reading your paper can go back to the source and locate exactly where that quote or idea came from.
For example:
Coca-Cola was originally invented by an American pharmacist to be used as a stimulating tonic (Standage 5).
The in-text citations in parentheses point readers to this reference in the Works Cited list:
Standage, Tom. A History of the World in 6 Glasses. Walker and Company, 2005.
In the above example, a reader can see that the information in the first sentence can be found on page 5 of Standage. When they look in the References for "Standage" they see the citation that lists the book in which you found the information. Now the reader can find that book, go to page 5, and see the information for themselves.