From what I can remember, an annotated bibliography is a list of citations to books, articles, and documents. Each citation is followed by an annotation, a brief paragraph, highlighting the relevance of the citation. Good annotations summarize the source you’re citing and also note why this source is important. So often annotations are two relatively short paragraphs that have 3-5 sentences each. That way, when writing out the outline, it’s easy to organize sources based on the paragraph on why the source is important to the research.