Definition of milk
It is also called "milk Player". When you "milk" a player, it means you have a very strong hand and you give him a friendly prize, a prize he can't refuse. I have if QQ on Q76Q8 and I bet milk 10% of the pot size on every street to make sure my opponent calls, I call milk. The fact of milker is betting very little on every street instead of checker, or betting standard, to maximize the size of the prize pool won.
Milk and metagame
If you are known as a player who likes "milk" his opponents, you might try an metagame game. If you are playing against a player who knows you well and knows that you always have a very strong hand when you bet very small on the river, you could use that metagame to bluff him on the river by betting small, as if you had a very strong hand. If your bluff works, you can show him so that the next time he sees you betting small on the river, he won't be sure if you are bluffing low or milk. It's an interesting metagame and you swing your range well.