The forest consists of dead trees located in a barren land. It is unknown why it represents pain, as its name suggests.
There is nothing else known about the Forest of Pain other than the events shown in Secrets of the Masters. After encouraging Rhino, Ox, and Croc to work together in stopping the Wu Sisters, Oogway escorted the trio to Hubei Volcano where the sisters' fortress was located. They walked through this forest along the way.