Purpose of this plugin is to make on site search engine optimization simple.
This plugin is intended to make on site SEO very simple, not necessarily easy. The basic idea is to add the meta keywords and the meta description tag to your pages and posts.
Next to adding the meta keywords and meta description tag it also has the option to override the default title of your post and/or display clean titles without the site name.
More information about the background of the plugin in the IFS SEO Simple main screen. Just download and install. The plugin won't change anything before you configure it, so you can install it safely. You will only see some additional fields in the edit screen, but if left empty they don't do anything.
And no, the keywords meta tag is not one of the two most important items in a webpage.