As part of the personalization team, I"m pretty addicted to looking at my search history for interesting patterns. So I decided to go a step further and write a script to pull together some stats about how I was searching. We thought other people might like to see this sort of thing too, so today we launched a Trends feature that gives you a look at a list of your top searches and clicks and other info about your search activity.
Official Google Blog: Setting trends
As Yu Chen mentioned in Official Gooble Blog, actually, it is just relevance keywords based on history data.