Amazon Best Sellers Rank (BSR) is a number that any product on Amazon is given once it has made at least one sale. All products are ranked based on how many units of it have been sold in the very recent history.
The lower a book’s BSR number, the more copies it sells every day. And vice versa – the higher the number, the less it copies it is selling.
For example, a book with BSR #2000 will be selling dozens of copies a day, whereas a book ranking at #120,000 will only be selling around 1 copy per day.
A mistake that new self-publishers make is confusing Best Sellers Rank with a book’s rank within a sub-category.
Whilst both rankings work in a similar way, they are different and should not be mistaken.
Amazon ranks products within separate categories, not against all products. Books (print books), Kindle eBooks, and Audiobooks all are ranked separately.