The theme has Twitter Cards enabled. To make sure the cards will fetch the description, too, you have to enable the search description.
- Go to Settings
- Scroll down to "Meta tags"
- Check "Enable search description"
- 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).