To simplify what is an absurdly complicated topic, the US Tax Code gives you a lot of "breaks" for certain things. For a lot of those things, the government won't know they happened unless you tell them (and you prove it). In most cases (i.e. for regular salaried or hourly employees) your taxes are automatically deducted, and then at the beginning of the new year you basically give the government a list of things that happened that could/would/should reduce your taxable income, which results in you getting money back from the government (sometimes).