This feature is part of our Professional plan, and allows you to analyze keywords mentioned in your survey comments, and place these in to different themes so that you can easily identify areas that are going well, and those that need a bit of work.
The theme buckets section is on your Dashboard page, just under the leaderboards section. To add a new Theme bucket click the Add button next to the title, or to edit just click on the pencil icon on the right hand side of exisiting theme.
You can add in any words that are related to that theme, and hit save! This will analyse all your comments (within the filtered date range), and you can see a breakdown of the NPS for each of these themes.