A keyword is a word or a short phrase describing the topic of a webpage. In practice, a keyword is the search query that you type into the search engine before hitting enter and seeing what it pulls up.
When someone searches Google for, say, “men’s basketball shoes” Google tries to discern what the user is looking for, and it returns a list of web pages that address that topic. If your website uses the exact phrase the user searched for as a keyword, part or of the phrase or a related phrases Google is more likely to show the user your website.