Meta tags were developed to assist with the rapid growth of web pages. In the late 90’s there was a major occurrence. Many Webmasters, generally those who ran adult-orientated websites, began to abuse the use of Keyword Meta tags. Many unrelated keywords were placed on their sites in the Meta tag section, causing their pornographic sites to begin appearing in search results unrelated to topics such as "Smithsonian". Thanks
Google can't give much important to keyword meta tag but we should keep on web page because of other search engine give priority to meta tag as tanmay says so many people abuse the meta keyword tag.
idea behind keyword meta tag was to give the search engines ideas on the content of your site. so if you put 3-5 keywords on your meta tag, search engines would assume that thats what your site is all about and would rank your site higher for those keywords. this is however old school seo and this meta tag has been abused so much and considering the fact that the site owners can put whatever they like on the keyword tag, search engines stopped paying attention to the keyword meta tags years ago.
Meta tags plays very important role in SERP for your targeted keyword for your website. Meta description is one of them.
keyword meta tag it give's the idea to the content and search engine will base and give priority on your site to higher the rank
Well keywords meta tags are irrelevant now a days.Yahoo has removed the use of the tag from its ranking algorithm.Google and Bing do not support the meta keyword tag. In fact, Google has never supported it.It do support several other meta tags like "description" meta tag.Use your specific keyword phrase just once within the description tag.It easily provide list of your primary keywords to your competitors.