When you read actively, you are participating in an ongoing conversation with the author in which you respond to and question the text. The notes that you take while reading document this dialogue. Your active involvement through note taking will keep you from getting distracted and missing important points.
If you own your book, you can mark text by highlighting, underlining or circling important parts that you might want to return to later. These may include main ideas, examples used to support the argument, illustrations of important points or sections where the focus of the argument shifts.
For more ideas on how to mark your text, see While You Read: Strategies for Close Reading.
Your involvement as a reader is not limited to highlighting and underlining the author’s words. Writing your own comments in the margins as you read is a process called "annotation." This is your chance to reflect on and challenge the meanings of what you read; record your questions and objections. Draw comparisons and comment on discrepancies.
Annotation also includes writing down definitions of words or terms that you come upon that are not already familiar to you. You also may choose to rearticulate or summarize an idea or argument to clarify it.
Annotating helps you to remember and internalize the material you read. It also encourages you to reflect on the text, and come up with your own ideas, perhaps for future essays.