Chapter 10 of the book High Performance MySQL discusses the importance of indexing in databases. Indexing is a way of organizing data in a database to make it easier and faster to search for specific information. The chapter explains the different types of indexes, such as B-tree and hash indexes, and how they work. It also covers the best practices for creating and using indexes, including choosing the right columns to index and avoiding over-indexing. The chapter concludes with a discussion on how to monitor and optimize indexes to ensure optimal database performance. Overall, this chapter is a great resource for anyone looking to improve their understanding of indexing in databases.