Top Rank is the rank of the term (or terms, if they have the same top rank) that has the highest rank out of this URL's terms.
Top Rank appears in the URL's top right, near the T icon.
You can filter URLs by giving a range (From-To) criteria for their Top Rank.
The "Top Rank" that appears in a URL's Overview & Daily page is different.
You will see historical term positions for a certain period. It's the highest rank that the term ranked in any one day in this period.