category. It relates to the number of sales a product has in an hour. The more sales it gets, the lower the sales rank. An item with a sales rank of 1 is the best-selling item in that category.
A chart of sales rank examples is given here. It’s hard to gauge how many units of a product you could sell based on sales rank alone, because the most popular item can sell hundreds of times more units than the second most popular item in a category (think Nobel Prize Books the day of the award ceremony). Good sales ranks usually also come with a lot of competition, meaning it may be less profitable to sell items with excellent ranking.