The idea
Counting sevens
A grid of digits. Count the 7s. It takes a while, and you have to scan every position.
Now the same grid with every 7 printed in orange, and bigger. You do not count any more; you just see them. Nothing about the data changed. Only the encoding did.
That difference, between scanning and seeing, is what this chapter is about, and it is the reason the professor opens with this exercise.