Similar to Facebook Open Graph Tags, Twitter Cards are a type of structured data that can be placed on your page to control what is shown when your page is shared on Twitter. You can indicate exactly what content should appear in a sharing snippet's title, description, imagery and other information.
We recommend defining as many of Twitter's Cards as possible, and inserting this code into the HTML of your page. Twitter has a cards markup tool for creating this content, or sometimes it can be written automatically with the help of a CMS plugin.