Bluetooth is one way in which tablets can wirelessly communicate with other devices, with others being the all-important WiFi and the, rare in children’s tablets, Near Field Communication (another way is to use wires via the tablet’s ports).
Bluetooth is not an essential communications method, and is comparatively uncommon still in children’s tablets, but it does give the ability to communicate with external devices such as wireless speakers. Bluetooth is not essential, but it can be a useful extra.
The nabi Jr. does come with Bluetooth 4.0, giving it an extra way of communicating with devices. In order to preserve battery life, though, it can be a good idea to turn off Bluetooth when it is not needed.