A keyword rich domain is a domain name which include a keyword related to targeted search queries. As keyword presence in domain names impacts Google ranking, the goal of keyword rich domain is to favor Google SERP rankings.
A keyword rich domain has for second benefit to favor clicks in SERPs as users identify the site domain name as being related to their search and because the keyword is bolded in the search result.
Google has diminished the importance of keyword rich domain names in search results but it has still some impact.