BSR stands for Best Seller Rank. "The Amazon Best Sellers calculation is based on Amazon.com sales and is updated hourly to reflect recent and historical sales of every item sold on Amazon.com. While the Amazon Best Sellers list is a good indicator of how well a product is selling overall, it doesn't always indicate how well an item is selling among other similar items. Category and subcategory bestseller lists were created to highlight an item's rank in the categories or subcategories where it really stands out." - Amazon.com

Check out our in-depth blog post about understanding BSR here