From the site's "How It Works" section:
We Give Books is a free website that enables anyone with access to the Internet to put books in the hands of children who don't have them, simply by reading online.
Simply choose the charity you want to read for and then select the books you want to read. For each book you read online, we donate a book to a leading literacy group on your behalf.
The more you read, the more we give.
That's right. You can help provide books to disenfranchised children by reading online. The books available to read are children's books, which makes this an especially great program if you're looking to read something to your child.