Bluetooth is a technical standard to ensure that separate devices are able to "speak" or to connect with each other. It's used widely to enable smartphone utility purposes.
To use Bluetooth on a phone open "Settings". Open Bluetooth and observe the Bluetooth status. It will show you which Bluetooth devices are currently connected to and will search for other nearby Bluetooth devices.
You will only need to keep Bluetooth switched on whilst you are using a wearable device such as Fitbit.