An annotated bibliography is simply a bibliography page (list of sources) in which each citation is followed by an annotation ("note") that describes or evaluates the source (or both). Annotations let your reader get a little extra insight into your sources and how they support your research.
Explain the main points of the work briefly.
Your summary should be original to you -- don't just copy/paste the publisher summary of a book or the abstract from a scholarly article.
The exact details of what to include will depend on your assignment -- number of sources, what to discuss in the annotation, and length of your commentary.