The purpose of writing an annotated bibliography is — oh, I’ll just let OWL at Purdue explain it:
A bibliography is a list of sources (books, journals, websites, periodicals, etc.) one has used for researching a topic. Bibliographies are sometimes called “references” or “works cited” depending on the style format you are using. A bibliography usually just includes the bibliographic information (i.e., the author, title, publisher, etc.).
An annotation is a summary and/or evaluation.
Therefore, an annotated bibliography includes a summary and/or evaluation of each of the sources.
If you click the link up there, you’ll find a more elaborate explanation of what sorts of things you might write in the summary/evaluation. Remember, we are interested in how you used the sources.