What role does vocabulary play in learning to read?

When children learn to read, they begin to understand that the words on the page correspond to the words they encounter every day in spoken English. That’s why it’s much easier for children to make sense of written words that are already part of their oral language. While we don’t have to know every word on the page to understand what weare reading, too many new or difficult words make comprehension impossible. As children’s reading level improves, so does the number of words they need to know.

http://www.nwrel.org/learns/resources/toolkit/NationalReadingPanel_FAQ.pdf