Form W-4, Line 5...Total number of allowances you are claiming allows the government to use your money instead of you.
If you are writing in 0 (zero), you are letting the government use YOUR money. When someone says, "Look at all the money I got back on my taxes." That isn't a good thing. It was YOUR money. The government was using it instead of you. On line 5, 0 (zero) takes out the maximum from your paycheck. Most people should put 1 or 2. You won't get that big tax return, but do you want YOUR money to work for you or the government?