A wolf tone, or simply a "wolf", is a noise that is produced when a note played on a stringed instrument matches the natural resonating frequency of the instrument, producing a tone that is loud and harsh, and basically unwelcomed by most musicians.
Wolf tones are usually only noticed on bowed instruments, most notably the violin and surrounding family, since the tones produced are played for much longer periods, and thus are easier to hear. Frequently, the wolf is present on or in between the pitch E and F.
The wolf note eliminator is designed to stop this unwelcome sound.