In audio terminology, especially car audio, the Head Unit is what the rest of the world will just all the Stereo, Tuner, Receiver, Radio or whatever. In any case, it is the part of an audio system to originate a/the signal; the first part of the system (head == first, right?).
In car audio (again, where this term is most used), head units (cum Car Stereo) will also commonly have small integrated amplifiers as well as the media player (Tape, CD, MD, etc) and radio tuner, though in the majority of "high end" setups a separate (and larger) amplifier will be added.
So, the head unit creates an electrical signal, the amplifiers, erm, amplify it and the speakers (also called drivers in professional parlance) turn the electrical signal into human audible sound.