Annotated bibliographies are common assignments because they require you to do the research for a paper without having to actually write the paper. Instead, you will create a properly formatted list (bibliography) of the information sources you have selected on your topic, along with critiques or summaries (annotations) of how each supports your research.
The annotations can be as short as a couple of sentences or as long as a couple of pages, depending on what your instructor requires. Typically, they expect a paragraph or two. Ask your instructor for specifics.
Here is the library's Annotated Bibliographies guide.
You may also use the Assignment Calculator, designed to walk you through the process step by step.