"Tree fern", as a mounting- or potting medium, is the dried-out trunk of any number of tropical tree ferns, often of the genus
dicksonia. It is a coarse, wiry, fibrous material that doesn't absorb a great deal of water, but does hold some in the interstices between fibers.
In some parts of the world, the ferns have been harvested almost out of existence - there are probably some species already wiped out.
A Google image search turned up this:
http://wow-orchids.com/images/sarcochilus2.jpg