A device comprised of a volume of air and an opening to the “outside.” The internal volume of a speaker cabinet and its port is an example of a Helmholz Resonator. A bottle is another example. Blowing air across the opening will produce a tone because of the air resonating, and the pitch of the tone will be related to the resonant frequency of the volume. In a vented (ported) speaker enclosure the back wave of air from the driver is used to reinforce the front wave at the resonant frequency. This phenomenon is commonly employed to extend the low frequency range of the speaker/enclosure system.
Helmholz Resonators are also employed in acoustics. Enclosing a volume of air (in a box, for example) while allowing limited access to the outside through a series of holes or slits in the surface can create a resonant system that will absorb (or, more accurately, cancel) standing waves and problem frequencies that may be too prominent in a room. If you have one or two frequencies that are too strong in your room a Helmholz Resonator is a very effective way of correcting it.