Almost every social network uses embedded OpenGraph HTML tags to generate link previews so readers can preview page content before they click through to read the post. https://blog.kissmetrics.com/open-graph-meta-tags/

Test your meta tags:

https://developers.facebook.com/tools/debug
https://dev.twitter.com/docs/cards/validation/validator
http://www.google.com/webmasters/tools/richsnippets
https://developers.pinterest.com/rich_pins/validator/