Jekyll SEO Tag adds the following meta tags to the site:

* Page title, with site title or description appended * Page description * Canonical URL * Next and previous URLs on paginated pages * JSON-LD Site and post metadata for richer indexing * Open Graph title, description, site title, and URL * Twitter Summary Card metadata

While one could theoretically add the necessary metadata tags oneself, Jekyll SEO Tag provides a battle-tested template of crowdsourced best-practices.