Metatags allow you to provide information about your site that can be understood by search engines and websites where your store and products might be shared such as Facebook and other social media platforms.
Default meta tags can be set in Site Settings > SEO and Sharing
Defaults are automatically applied for Artists and Labels, but can be overridden under the SEO and Sharing section of an individual page, artist or label.
Note that Twitter cards are set automatically for product pages.
This is a short description of your store. Try to use your main keywords in your tagline. Take for example the Bleep tagline: “Your Source for Independent Music - Download MP3, WAV and FLAC, Buy Vinyl, CD and Merchandise”
This is presented in various places as the default page description for pages on your site.
Importantly this is used for the Google Description Metatags. For more information about Google Meta tags, check out the Google Support Documentation
You can set default keywords for your site. Google and many other search engines no longer rely on keywords and now parse the information that they find on your page directly, so consider this an optional field.