To control what content is displayed in social media posts, select a featured image for a post or page and then complete either the SEO meta tags or excerpt field. Then simply copy the URL of that post or page and paste into your social media post.
Note, these fields have multiple uses such as the featured image and excerpt field also being displayed when building a grid of posts using the show posts row. The SEO fields can be used to override the default content displayed in search engine results.