Good readers are able to read the text and determine what information is important. Decisions about importance are based on the reader's purpose, the reader's prior knowledge, and the reader's sense of what is interesting. When students are asked to read nonfiction materials they need to be able to identify the key ideas or themes. They use their knowledge of text features to make predictions about organization and content. Teachers must help students read strategically by setting a purpose for reading, which will provide students with guidelines for selecting what is important in text.
Websites on Determining Importance:
Determining Importance in Nonfiction by Stephanie Harvey - PDF
LikeToRead.com: Determine Importance
Determining Importance Lesson Plans
Determining Importance in a Math Text