An annotated bibliography is a list of citations, in a standard referencing style (e.g. APA), each followed by a short paragraph, called the annotation.
The short paragraph (usually 150-200 words) summarises and evaluates the source. Annotated bibliographies are usually arranged in alphabetical order by author.
You may be asked to write an annotated bibliography as a stand alone assessment task, or as part of a larger research report or academic essay.