A keyword, or a focus keyword as some call it, is a word that describes the content on your page or post best. It’s the search term that you want to rank for with a certain page. So when people search for that keyword or phrase in Google or other search engines, they should find that page on your website.
Every page and post on your website should have at least one keyword to help the search engines index your pages and improve your search results.
Source: Yoast: What is a keyword