Bestseller rank refers to a number that shows how popular a book is compared to other books. A low number means the book is selling really well and is considered a bestseller. Full definition
You can also see your sales by geographical area, and if you look at your sales Rank Over Time, you get the Amazon Bestseller Rank of all the books you've added to author central (this is a key point — don't assume it's all your books — it's only the books you've added, so if you forget to add a book to author central, it won't show up in the rankings). (tobecomeawriter.com)
As an aspiring thriller writer, I kept noticing the bold, brassy covers of books by one Mel Comley sitting high in the genre bestseller rankings on Amazon. (bidinotto.com)
3) If you're going for Genre Bestseller Rank by first making your book free and then switching to paid, then our 7 Day Push promotion package is a good choice. (booksbutterfly.com)