Theme related searches

I was reading Affiliate Masters ebook today and I went to check on some of the links. One of them came out pretty interesting and that is called 7 searches. Why did I say that? Stuart ever told me that I need to do theme related keywords, meaning to say that if you are targeting on a specific group of readers (niche), you need to understand what are the other words your targeted group is likely to search for other than the keyword(s) you have determined.