To increase your book's discoverability on Amazon, you need descriptions and keywords that accurately portray your book's content and use the words customers will use when they search. Along with factors like sales history and Amazon Best Sellers Rank, relevant keywords can boost your placement in search results on Amazon.com. 

Best practices with keywords: 
Combine keywords in the most logical order: Customers will search for military science fiction but not for fiction science military. 
Use up to seven keywords or short phrases. Separate them with commas, and keep an eye on the character limit in the text field. 
Experiment. Before you publish, search for your book's title and keywords on Amazon. If you get irrelevant results, or results you dislike, consider making some changes—your book will ultimately appear among similar results. When you search, look at the suggestions that appear in the Search field drop down. 
Think like your customer. Think about how you would search for your book if you were a customer, and ask others to suggest keywords they'd search on. 

Useful keyword types
● Setting (Colonial America) 
● Character types (single dad, veteran)
● Character roles (strong female lead) 
● Plot themes (coming of age, forgiveness) 
● Story tone (dystopian, feel-good) 

For suggestions on search keywords based on browse category, read more here. 

Do not include these things in keywords: 
● Information covered elsewhere in your book's metadata—title, contributor(s),  category, etc. 
● Subjective claims about quality (e.g. "best") 
● Statements that are only temporarily true ("new," "on sale," "available now") 
● Information common to most  items in the category ("book") 
● Common misspellings
● Variants of spacing, punctuation, capitalization, and pluralization (both "80GB" and "80 GB", "computer" and "computers", etc.). The only exception is for words translated in more than one way, like "Mao Zedong" and "Mao Tse-tung," or "Hanukkah" and "Chanukah." 
● Anything misrepresentative, such as the name of an author that is not associated with your book. This type of information can create a confusing customer experience and Kindle Direct Publishing has a zero tolerance policy for metadata that is meant to advertise, promote, or mislead. 

Don't use quotation marks in search terms: Single words work better than phrases—and specific words work better than general words. If you enter "complex suspenseful whodunit," only people who type all of those words will find your book. You'll get better results if you enter this: complex suspenseful whodunit. Customers can search on any of those words and find your book. 

Other metadata tips
● Customers are more likely to skim past long titles (over 60 characters). 
● Focus your book's description on the book's content
● Your keywords can capture useful, relevant information that won't fit in your title and description (setting, character, plot, theme, etc.) 
● You can change keywords and descriptions as often as you like
● If your book is available in different formats (physical, audio) keep your keywords and description consistent across formats
● Make sure your book's metadata adheres to KDP's Metadata Guidelines.

[From Booktown]