Your new website has been programmed with some very important Meta Tags. These include 'keywords' and a 'description.' Keywords or keyword phrases help to indicate to search engines what the important topics of your website are. These should be reinforced through repetition and prominence throughout your site's content. The description is what will appear within search engine results, and will help determine if a visitor will click on the link to your site.
You can and should revise and edit these elements of your site regularly, and can do so easily by going to Content Management -> Meta Tags.
Additionally, if you are regularly posting new content to your site, like Blog entries, you will find a Meta Tag section within the Edit Mode for that content. You should regularly add a few keywords to that field that are appropriate for that entry, yet might not be found in your site's global keyword list.
Do not repeat keywords, nor should you worry about plural words.