Entries in an annotated bibliography have two parts; a citation for each book, article, website, video, or other information source that you are consulting, and an evaluation of the source's content. Often this means more than a synopsis of the information provided, but some critical analysis of how useful the source is for your project and soundness of the argument made by the author.
Here is an example of an annotated bibliography entry for a journal article from the Cornell University Libraries*:

Here are some more resources that discuss writing an annotated bibliography:
*Reproduced with permission of Olin Library Reference, Research & Learning Services, Cornell University Library, Ithaca, NY, USA