The plugin adds a meta-tag to the html-header-part of your blog. This meta-tag contains keywords that describe the contents of the html-page. Adding this kind of meta-tag is one part of the job of optimizing your blog for search engines. To learn more about search engine optimization, I recommend reading and understanding these articles:
Just download the zip-archive, extract the plugin-file and upload it to
/wp-contents/plugins/wptags-4-metakeywords.
Then go to the plugin-admin-section of your blog and press activate. Now go to the options section and click on �WPtags 4 MetaKeywords�. There, you can enter some default-keywords, that are used on pages, where the plugin cannot determine automatically, which keywords to choose.