Its a large brush with very flexible bristles that are on a spongy base.
The flexible bristles & the movement of the spongy base soak up the friction caused by brushing the hair and absorbs the impact.
I've made it sound very technical but its not really.
Its basically a large (usually square) hair brush that is more gentle on the hair.
You can buy them from anywhere that sells hair accessories & products.
The important thing is that the brush is soft and preferably the ends of the bristles are blunt ended.

Have a look at these:
http://tinyurl.com/677o6hk