What are they?
If you have never had to write an annotated bibliography before, it might seem overwhelming. An annotated bibliography is simply a bibliography with notes.
Why do we need them?
What purpose do they serve? They help you to keep track of the research you've done while preparing a project, and allow you to reflect on the work that others have created around your topic. After you've read a lot of sources, you'll want a quick way to remember what each of them was about and how they help your overall argument or thesis. Annotated bibliographies help you create little summaries to refer to later, while also acknowledging that you're familiar with important and relevant works in your field.
Although they take time to put together in the beginning, they save you time in the end.
How? If you have a research paper or project to do, you are not going to remember every single article you've read. Sometimes even if you do remember, it's hard to find the specifics when you need them.
By making an annotated bibliography, you can quickly hunt down the article you want to cite from, remind yourself of what you've read, and have your citation already typed up so you can just paste it into your final Works Cited page.
This video is great if you get stuck or want an alternative explanation. It has a second part, a third part, and a fourth part.
This is another great place to look.