It's a treasury for promoting and sales, "BNS" stands for Buy and Stay. The first 15 spots are free. Then to get a feature in the next round, you have to buy in by making a purchase from a featured seller on the BNS. When enough sales have been made to have enough featured shops, the featured shops are replaced and the next round starts.
A lot of times people chat in the comments and list coupon codes for their shop when featured.
This post explains BNS well:
http://www.etsy.com/teams/10875/bnr-and-bns/discuss/9235312/page/1/