Wolf tones are hyper-resonances that often occur with cellos. On some instruments they present no problem, but on others they make certain pitches difficult to play well. They typically occur in the area of Eb-F# in 4th position on the G string (as well as the same pitches higher on the C string). If you are looking for a cello to buy or rent that has wolfs on the D or A string, just move on, this is not worth it.
With most wolf tones, it is possible to treat the wolf such that it is quite manageable. Recently there have been some new developments in this area.
The most common sort of wolf eliminator fits onto the string between the bridge and the tailpiece. In using these, first try it on the C string and tune it to the pitch a 5th above your wolf tone. You tune it by bowing the string between the bridge and the wolf eliminator. If that does not work, then try a 4th. If that does not work, try the note itself. The further away from the bridge you get, the fewer overtones you kill, which is why it is better on the C than the G. (There are 3 octaves of pitches on the other side of the bridge, BTW). If this is not effective enough, then try the G string.
Many people find the new Krentz eliminator much more effective and less problematic for the sound. Some people have reported that it can actually improve the sound of the cello.
Krenz Wolf Eliminator