The Brush is a tool used in Harvest Moon: Tree of Tranquility.
After you have animals in the barn, the brush can be purchased from Brownie Ranch for 1200. Equip the brush, stand next to an animal and press the 'A' button to brush your barn animals. This can be done once per day per animal in order to increase their affection.
Brushing is the fastest way to increase an animal's affection, so the brush is a very important tool in order to maximize profit from your animals and get the best quality products possible.