An annotated bibliography is a helpful research tool that lists useful resources for a specific research topic or area. It includes a short commentary for each book, article, Web site, etc. that it lists. The commentary can be descriptive (tells about the content) or evaluative (tells about the author's point of view, whether the item is "mainstream" or takes a new or radical tack, and for what kind of research it would be useful). Often, it is both!
Annotated bibliographies answer the question: "What would be the most relevant, most useful or most up-to-date sources for this topic?" You can see how they are of value to scholars. And, as an assignment, they get you to engage in the work of scholarship, using your knowledge, research, and critical thinking skills.