A keyword is a specific word or long tailed phrase. The keywords of a web page are there to sum up the content. These words are intended to be a quick way to sum up an entire website page. For example, if your site is about Nike. You are not going to have keywords relating to grilling. Some examples of keywords for a site about Nike are shoes, clothes, basketball, golf, etc. Keywords are used within a sites metadata. This metadata helps search engines match the appropriate search query to the specific web page.
Previously in search engine optimization, keywords played a vital role in ranking your website above the competition. Search engines would crawl sites, to simply see if the site's keywords matched the searched query. Therefore, SEO analysts would take note of this, and abuse the system. Leading to the importance of meta keywords to be significantly lowered. Keywords still arguably play a vital role in SEO ranking today, but they are no longer a major priority to SEO analysts.