These are the tags for your book that will help searchers (potential customers) find it. Entering in keywords is optional, though I have no idea why you wouldn’t want to add them. These keywords can be words or phrases. Amazon recommends that the phrases be 2–3 words long. Amazon also gives a good guideline for choosing these words and phrases: “To determine the right search keywords for your book, we suggest you think like a reader browsing for a book with your book’s topic. What are your target readers searching for?”
Per Amazon from the link above, you are not allowed to include any of the following in your keywords:
• Reference to other authors
• Reference to books by other authors
• Reference to sales rank (i.e. ‘best-selling’)
• Reference to advertisements or promotions (i.e. ‘free’)
• Reference to anything that is unrelated to your book’s content
I don’t know how stringently Amazon patrols or polices these keywords, so you might be able to get away with something from the above list, but do so at your own risk.
Amazon has a great article called Make Your Book More Discoverable with Keywords, and it gives some helpful tips for coming up with keywords.
To reiterate, the most important think to keep in mind here is to add keywords and phrases that reflect what people are actually searching for. If you don’t know what order to phrase something in, use Amazon’s predictive search to help you out. You know how when you’re on Amazon and you enter in the first couple letters of something you want to find, and then Amazon provides a list of things that start with those first couple letters that it thinks you’ll want to search for? Use that to your advantage. Let’s say I wanted to write a book about rock climbing for beginners. Should my keyword be “beginners rock climbing” or “rock climbing beginners”? Well, if I type in “beginners rock,” the suggestion I’m given is “beginners rock climbing shoes.” Not super helpful. But if I type in “rock climbing be,” one of the suggestions is indeed “rock climbing beginners.” This may not be the best example, as “rock climbing for beginners” would probably be the very best choice, but at the very least, you know that if a word or phrase is provided in the predictive search box, someone out there is searching for it.
One more thing to keep in mind is that some categories have keyword requirements. If one of the categories you choose is Science Fiction & Fantasy/Horror, for example, you are required to include the keyword “horror” as one of your 7 keywords. You can find more information about these requirements partly down the page on this page from Amazon.