Configure global meta tag default values using your site's global Metatag information. Meta tags inherit from one level to the next unless they are overridden. To view a summary of the meta tags and the inheritance for a specific configuration, click on its name in the Type list provided in the Metatag section.
Meta tags can be further refined on a per-entity basis, meaning on each content type (Article, Basic page, Event, etc) you can specifically adjust metatag information under the Additional options section located on the right side while in Edit mode, under the SEO tab.
To learn more about writing good meta tags and the influence they have on search engine results, consider starting with this article: "How to write a meta description that gets click-throughs" by Neil Patel at Search Engine Land, and then search the web for more resources.