It's important that you test your code and your themes, but that takes time. Luckily there are tools and methods of simplifying and automating these things. This chapter is going to cover basic preventative testing, and tools to help catch bugs and things you may have missed.
Good for testing content
It's a content export file
Contains lots of posts and categories of varying types to test as many possible combinations as possible
Useful for testing themes and unhandled scenarios such as posts without titles, giant nav menus, or very long tag names.
Good for testing theme completion
A plugin that runs several automated tests on the current theme
Checks for things the theme review team checks for when submitting themes to wordpress.org
Includes things such as comment forms, showing tags and categories, displaying author names, etc
Good for testing code and as a development methodology
Explain the difference between the three
Mention they're covered in more depth in sub-chapters