On / Off (Bluetooth)
The main On / Off switch for Bluetooth settings enables or disables Bluetooth support as a whole. While off, you can't use any Bluetooth connections with your device. If you use a Bluetooth headset, or a Bluetooth connection to your car or home stereo, for example, these will simply not be connected and your phone will treat them like they are not in range or turned off.
Bluetooth does not use a significant amount of battery power while it is enabled but no Bluetooth devices are connected; however, if you never use any Bluetooth devices it will be worth turning this off for the minimal benefit it is likely to bring.