It's exactly what it says...horsehair that has been coated with a thin coat of some sort of adhesive (rubber) that also has some small "blobs" of the adhesive attached, which give the appearance of leaves and such. It comes in blocks or strips and is difficult to find in the U.S.
It is primarily used to replicate tangled ground cover or undergrowth for hedges, bushes, etc.
I good book that describes its use in detail is "Terrain Modeling" by Richard Windrow and is part of the Osprey Masterclass series - ISBN # 1 84176 062 5.
I got mine through the Military Book Club, but there are several other sources as well. It's a really nice book (IMHO) and covers a broad range of groundwork modeling.
Hope this helps you out. Happy modeling!