An Annotated Bibliography is a list of sources, formatted in the documentation style of preference, where each source is followed by a paragraph about that source. This paragraph is the "annotation" for that article.
Many annotations summarize the content of the cited article, while others are intended to assess or evaluate the source.
Before writing the annotation, you should establish what kind of annotations are required for your assignment.
Assignment sheets or instructions from your professor typically let you know what your focus should be.