What is "Bluetooth Low Energy"?
Mobile devices can do more and more things today, and ship with more and more powerful hardware. One thing has a hard way to keep up with this: the mobile device's battery. Its capacity doesn't grow with the same speed. One way to cope with that is making new hardware less energy-hungry. For this, the Bluetooth 4.0 specification introduced a new feature: Bluetooth Low Energy. The Wikipedia article1 puts it like this:
Bluetooth low energy (BLE) is a feature of Bluetooth 4.0 wireless radio technology, aimed at new, principally low-power and low-latency, applications for wireless devices within a short range (up to 50 metres / 160 feet -see table below). This facilitates a wide range of applications and smaller form factor devices in the healthcare, fitness, security, and home entertainment industries.
When should I use this tag?
If your device already supports Bluetooth 4.0, announces BLE compatibility, and you are having issues with it, the bluetooth-le tag is the one you should use.