Clean Code by Robert Martin is a guide to writing software that is easy to read, maintain, and extend. The book argues that code should be clean, meaning that it should be well-organized, well-documented, and free from errors. The author advocates for writing code that is easy for others to understand, even if it takes longer to write, as it will save time in the long run. The book covers topics such as naming conventions, functions, and object-oriented programming, and provides practical examples and case studies to illustrate best practices.