Writing an annotated bibliography is an excellent way to prepare for a research project. Just collecting sources for a bibliography is useful, but when you have to write annotations or summaries for each source, you're forced to read each source more carefully.
You begin to read more critically instead of just collecting information--and that's a good thing!
An annotation is a summary and/or evaluation. Depending on your project or the assignment, your annotations may do one or more of the following:
Summarize: Some annotations merely summarize the source. What are the main arguments? What is the point of this book or article? What topics are covered? If someone asked what this article/book is about, what would you say? The length of your annotations will determine how detailed your summary is.
Assess: After summarizing a source, it may be helpful to evaluate it. Is it a useful source? How does it compare with other sources in your bibliography? Is the information reliable? Is this source biased or objective? What is the goal of this source?
Reflect: Once you've summarized and assessed a source, you need to ask how it fits into your research. Was this source helpful to you? How does it help you shape your argument? How can you use this source in your research project? Has it changed how you think about your topic?