I’ll give a very basic definition. Search engines, like Amazon’s A9 or Google, rely on keyword indexing to help them decide what search results to show for a particular search query. A9 or Google essentially creates an index of keywords and what product pages are associated with that keyword. With A9, Amazon, basically “reads” your product page and notes or indexes the copy or keywords you have used to describe your product. Each keyword is counted once and more weight is given to keywords in the title.