The Milker is a tool used in Harvest Moon: Tree of Tranquility.
The milker can be purchased from Brownie Ranch for 1500. It is used to milk adult cows. Equip the milker, stand next to an adult cow and press the 'A' button to milk them. This can be done once per day on each healthy adult cow.
Using the milker will allow you to gain affection from your cows as an added bonus.