The book explains refactoring: how to keep the internals of a program in as healthy condition as possible, so that inevitable changes are faster and easier to make, and so that the quality of the software is higher.
Most software projects really try to do their best to produce good software, by using all the best practices they can to get a good design, and the best programmers they can find, and so forth, but slowly, but surely, almost every program gets in a worse and worse state.
Refactoring deals with how to stop programs ever getting in such a bad state, and even deals with taking programs which are in very poor condition and making them structurally very good again.