keywords are basically the topic of your website or webpage. For instance, if you write a webpage about "The Best Rock Bands Of All Time" it would make sense that you choose keywords to use appropriately throughout the page such as rock bands, best rock bands, rock music, etc. There is much more to learn about how to optimize your content effectively, but that is a basic idea of keywords.
Keywords are the words searchers or people using the internet use when searching for information or products on any search engine. They are what search strings are matched against. Read on about this from www.webconfs.com/seo-tutorial/seo-and-keywords.php.