What is this supposed to do?

What is example 06B (in the book: Getting Started With Arduino by Massimo Banzi) supposed to do?

Not having the book, could you describe what it is? :P

Set the brightness of LED to a brightness specified by the value of the analogue input [in this case your analogue sensor is a light-dependent resistor (LDR) or photoresistor]

It says up at the top of the sketch what each example number is and what to expect the sketch to do (a good habit to keep and use in your own sketches).