Meta Defaults allows you to set the Standard Meta tags for pages.
Meta tags are used to provide information to search engines about what the page is about and are used in Search Engine Optimisation (SEO).
Each page can be assigned its own meta tags but these are the defaults for the site.
Page Title - Default title, should be one short line describing the site
Description - Default description, between 1-3 sentences describing the site.
Keywords - Default keywords, the main keywords relevant to your site