What is the "itch" of which Beatty speaks?

The "itch" Beatty speaks of is curiosity. Beatty referenced this tendency also when he complained that Clarisse wanted to know "why" things worked instead of "how." Curiosity indicates a desire to learn and to know, which, in Bradbury's society indicated dissatisfaction with the status quo. To "scratch" the itch was to read. Beatty discouraged that by telling Montag that books were full of nothing.