Enable Twitter cards

The theme has Twitter Cards enabled. To make sure the cards will fetch the description, too, you have to enable the search description.

1. Go to Settings
2. Scroll down to "Meta tags"
3. Check "Enable search description"
4. Click on "Search description" to add a description for your blog

You'll have to add a description for each post, too - open a post in the editor > click on "Search Description" from the right column (below "Location") > add your text (it can be the first 1-2 lines of your post; there's a limit of 150 characters).