You can see the explanation in the tooltip of the “top” menu. It says “the most active topics in the last year, month, week or day”.
https://github.com/discourse/discourse/blob/master/app/models/top_topic.rb#L252-L256
As you can see Discourse calculating topic score for all the periods using the counting of posts, views, topic likes and post likes. And topics will be displayed in descending order of the PERIOD_score.
@sam I just noticed quarterly_score index is missing (or no need?) here while all other periods having their own score index. Yes I already know it will not affect the result.