In order to configure Twitter Cards Tags for Magento 2 CMS Page please go to Admin Panel > Content > Pages.

 Twitter cards admin panel

Then choose a page you are interested in and start editing it by pressing the Select button.

Page list editing

Find Twitter Cards Metadata section. 

Twitter Cards Metadata

1. Set a Twitter Cards Title.  Meta Title will be used if you don't set it up. If it isn't configured either Page Title will be used.

2. In the Twitter Cards Description set the description of the page. In case you haven't set it up the Meta description will be used. Default Description will be used if there are none of the descriptions configured.

3. Set the Twitter Cards Image, otherwise, Default Twitter Image will be used.

 Twitter cards tags configuration on cms page

Once you have finished don't forget to press the Save button.

Magento 2 Twitter Cards Tags Configuration, CMS Page, Save