The BT500 Bluetooth speaker looks to be exactly what the name says, a Bluetooth speaker. Like the many, many, many other similar speakers on the market, the BT500 will connect with the Nook HD and Nook HD+ so you can stream music and not be stuck with the small speakers on the Nook HD+.
It should also work with almost any compatible BT-equipped smartphone, tablet, or other portable electronic device. There's no word yet on when it will be released or how much it will cost, but it is already listed on the B&N website.
This is Barnes & Noble's second Nook-branded audio accessory. The first was a set of $30
$50 headphones, which should tell you something about B&N's ability to develop hardware.