On page 15, we read:

In this example we’re sending a function, price -> price * 0.9, as an argument to mapToDouble The function being passed is created just in time, at the point of call to the higher-order function mapToDouble.

As a simple clarification, one might write:

In this example we’re sending a function, price -> price * 0.9, as an argument to mapToDouble The function being passed is created just in time (at least conceptually - it really is compiled into executable code and a class (if necessary) at compile time), at the point of call to the higher-order function mapToDouble.