Temporary variables are a integral part of any code. But a splattering of the same all over can make your code hard to understand or modify. Replace temp with query is a refactoring method where you replace temp variable expressions with methods. This method is often also required before you use the Extract Method refactoring.
In his example, he takes a variable inside a current method (base_price) and replaces it with a method call by the same name resulting in a more reusable format other methods can call rather than just computing the value themselves.