A buzz box is used to apply a current load on a wire. A wire that is hanging on by a couple threads will check good with a ohm meter, but it might not be able to carry the required load.
A buzz box can be built to apply almost any amperage. The box shown here will put approximately 1 amp across a wire. A 22 gauge or larger wire can safely handle 1 amp for testing. 24 gauge wires, which are about the smallest installed on aircraft, are mainly used for signals. Putting 1 amp across a 24 gauge wire is not a good idea. On the rare occasion where a larger wire needs to be loaded, aircraft bulbs used with 28 volt battery power are a good way to load a wire. Medium sized bulbs such as a runway turnoff or wing illumination light will put a hefty load on a 14-16 gauge wire.
The buzz box or light is going to accomplish one of two things, either the wire will carry the load or it will burn through. If a wire goes open, it is easier to find and correct the problem.
All the components for this box above were purchased from Radio Shack®. The resistors, light, and sonalert are wired in "series". The wire under test completes the circuit.