Twitter Cards helps you attach rich media to tweets that link to your content. When your tweet has a link to a site that has twitter cards enabled, there is an option to view summary. Whenever another user clicks on the tweet, they will see a summary of the post. Adding twitter cards in WordPress have two ways.
Method 1: Yoast SEO plugin
If you are using WordPress SEO by Yoast plugin, then you are in luck. Yoast has built-in integration for twitter cards, so turning it on is a matter of few clicks. Simply go to Social page of Yoast SEO settings page. Enter your Twitter username and save changes. Then, click on the Twitter tab under that you need to check the box for Add Twitter card meta data. Select a default card type to choose. Lastly, go to User > My Profile page. You will see a new field under Contact Info for your twitter username and your username there. This method is the easiest one if you are using WordPress SEO by Yoast.