Mixing down to base band
Baseband is low frequency signal that is to be sent across (eg: human voice in telephone). The usual procedure to send a baseband signal is to modulate it using a high frequency carrier in this case Bluetooth (or you can say mix it) i.e. in the range of 2.4 to 2.48 GHz. 

Baseband definition (also includes a part of modulation) : https://en.wikipedia.org/wiki/Baseband

This would further help you understand :