Citations might come from the bibliography of another article/book, a list of suggested readings from a textbook, a Prof, an online source, etc. They provide the information required to locate the document.
It's essential that you understand how to read a citation, to establish whether it's a book or a journal, and to identify the title of the larger work in which it was published. Where you go next depends on the type of document.
If it's a journal article, there are three ways to locate it.