A wolf note is a note on a musical instrument, or really anything, that makes the object resonate more than usual. Of course the instrument resonates some all the time, to create sound, but we're talking about unusually large resonance.
Resonance is powerful, it's why protesters can topple over a truck; by pushing and pulling when that action will add to the already occurring motion.
So the wolf note is a given note on an instrument that makes the whole instrument vibrate at that frequency, much louder than the other notes.
Or rather, that's how it works for instruments that aren't meant to work that way. Compare. for example, a pipe organ, a flute, and guitar. A pipe organ has cylinders of specific diameter and length related in such a way (it has to do with wavelengths and the speed of sound) to make the air in the pipes resonate at the note desired. Essentially, a pipe organ plays nothing but wolf notes. Except when the room it's in has resonances in the organ's range, that can get exciting.
The flute is of average diameter with relation to its notes, but the air column is of variable length. When the player blows over the hole, it's exciting the air column, forcing it to resonate at the frequency dictated by the length of the air column.
Then it's useful to look at the example of a guitar. The note a plucked string makes when it's plucked is dictated by string tension, string mass per unit length, and string length. When you use the tuning pegs on a guitar, you're varying the tension. When you switch strings to heavier strings, the mass per unit length rises, and the resonant pitch lowers. When you finger a fret, you're reducing the effective length of the string, raising its resonant frequency.
A wolf note comes along when a string's resonance and the resonance of the guitar as a whole are the same, or even-integer multiples of each other. One vibrating thing moves another. This creates uneven volume levels, and generally, the wolf note's character is different (the proportions of the overtones are different), and is generally undesirable. Although, with new music, who's to say?
The concept can extend to anything that makes variable-pitched sound, a car could conceivably have a wolf note at a given engine RPM. iDan's computer speaker has a wolf note around 2125 Hertz (aka Hz), due to resonances in his computer case. An ideal speaker would not have a wolf note, but in practice, many do. You may notice that the air cavity in your car has a certain wolf note in some music that you listen to, which disappears when you open a window, because the air is no longer captive, instead of the vibrations being contained, they're leaked to the outside. (Incidentally, a closed car is an awesome space for bass, the air cavity's just about the right size for it to be positively resonant in our audible bass. Tell that to the guys who're blasting the neighborhood next time (so they can blast you))
The term 'wolf note' is also used to describe the effect of raising the pickups too high on an electric guitar. This causes the magnets in the pickup to interfere with the string's motion, causing strange effects, sometimes warbling and sometimes just dissonance. Thanks to sideways for the info in this paragraph
Conversely, there are null notes too. Notes that are much quieter than the others. Same idea with dampening, instead of resonance. Most guitars have one or two dead or null notes.