Meta keywords are keywords that can be listed in the meta tags of the HTML source code. They describe in short, concise words the content of a website. Meta keywords can therefore serve as important indicators of a website's content to search engines. The keywords are generally written in lower case, and separated with a comma. Today, meta keywords are irrelevant for search engine optimization and don't have any ranking relevance.
<meta name="keywords" content="metatags, metakeywords" />
At the beginning of the commercial Internet age at the end of the 1990s, one of the first SEO measures was to enter many frequently searched terms in the field for the meta keyword in the source code, because the search engines of that time were dependent on this field in the meta data of an HTML website due to their technology. The keywords were used to assess the relevance of a search query in relation to an existing page in the search engine index, taking into account other factors. Many keywords therefore usually meant good rankings in the search results.
For example, if the meta keyword "download" is often entered on a web page, the page will be easier to find when searching for "download". The small hurdle of "filling" the keyword meta tag into the search results to bring web pages forward lead to an abuse of the tag. Search engines recognized early on that the tag was misused for manipulation of the rankings. For a long time, the tag has been considered a real Low Hanging Fruit of search engine optimization. However, it took until 2009 for Google to publicly confirm that the meta keyword tag is no longer relevant for the rankings. 
It is unclear whether Google and other search engines no longer consider Meta Keywords. Meanwhile, such meta keywords are even suspected to be the source of penalties being imposed on websites. This applies particularly if a large number of keywords were used in the source code which do not occur in the text of the webpage, or if some of them appear to be spam (such as “porn,” “poker,” “credit,” “download”). Today, in contrast to the Meta Description and Meta Title, the meta keywords are often no longer filled with terms.
To understand the use of meta keywords by search engines, we should distinguish between indexing, information retrieval, and ranking. That way, the differences in the weighting of meta keywords with respect to the ranking can be illustrated.
Meta keywords are considered as relevant by search engines only in a few cases. Google does not use the keywords listed in the meta area of a website for ranking. Even Yahoo and Bing use them only to a limited extent for the hierarchy of search results. In the latter, this information is only one of many factors that determine the order of the search results.
Webmasters can store meta keywords. But it is important here to ensure that their amount stays within reason and the keywords correspond to the actual page content, because meta keywords should not give any notion of spam or keyword stuffing to search engines. As a general rule, no more than ten meta keywords should be recorded. Less is more. The main keyword for a webpage should be the focus. Although synonyms and long tail keywords can be listed, it would be more advisable to use these in text visible to users and concentrate on the content. The quality standards of the individual search engines such as the Google Webmaster Guidelines will assist you here.
There is another important aspect connected with meta keywords. Any competitor can find out those main keywords by looking into the source code. If this is not desired, the use of meta keywords should be reconsidered. Some experts even recommend omitting meta keywords to prevent such risks from the outset. Especially since Google ignores them and most of the traffic for many site operators is most likely generated by this search engine.
Some CMS use meta keywords for the internal search. A search engine that is integrated into a website accesses the saved keywords during a search query of the website - the saved keywords can also be stored in the Backend. However, modern website searches work much more complexly and can present adequate pages to users on the basis of existing content without having to access stored keywords in the source code.