It's a special de-matting comb that is designed to break through mats without doing too much damage. I got mine at Petedge, but they have them all over. Here is the one I have ...
PetEdge: Master Grooming Tools Dematting Combs
Here is the link to "The Stuff" while I'm on that website ...
PetEdge: The Stuff Conditioner and Detangler