highlight the words as you move along reading a book or anything else online but books primarily.
You could load any online book into the app and the reader could use the spacebar or an arrow key to move a highlight square along the words as she read at your her pace. If you could also have the option to have the highlight box move along at different cadences by itself to help lead the reader that would be a bonus.
Jolyon Terwilliger commented
Check out readfa.st