The significant changes tool highlights themes which are changing over long time periods, and not month over month.
For a full explanation you can read How Significant Changes are calculated, otherwise generally Thematic looks at the last 6 time periods (Usually months) and then identifies if a theme has an increase or decrease in volume or score based on that 6 month average.
Click a time period on the score over time graph to see the themes with biggest changes in that time period. You can use the Score & Monthly to select a different score, or time period (Weekly, Monthly, Quarterly). There are 3 sections of themes displayed, click on a theme to see more details
1. Rare themes - Themes with 0-3% volume that are increasing. Can be useful to identify before they trend larger and becomes more impacting on the dataset
2. Themes by volumes - Themes with the biggest change in volume, can be useful for identifying themes with larger shifts over time.
3. Themes by average score - Themes with the biggest change in score, works with all available scores like NPS or CSAT. Useful for identifying themes which are becoming more positive or negative over time.