"My interest" shows you the type of websites you visit and it will display a keyword graph over time.
It will show categories like retail, travel, entertainment, news, technology & science, etc according to your interests. The category in bold represents your highest interest.
You can view how your interests have changed over time by selecting the time selector at the bottom of "My interest".
Companies and websites that use trackers collect similar information about you. If you want to limit the ability for those organizations to track your interests, you should consider blocking trackers with the Tracker Manager.