Bluetooth Low Energy (LE) allows you to connect your phone with special accessories like heart rate monitors. While doing that, thanks to Bluetooth LE connection, the accessory battery will last much longer, up to months.
It is already available for 520, 620, 625, and 720 with Lumia Amber. With Lumia Black software update, Bluetooth LE will be enabled across the full WP8 Lumia range.
Currently Nokia Lumia phones with Bluetooth LE feature enabled only support Adidas MiCoach accessories.