After the Tentative Tax has been assessed based off of your Taxable Income, your Total Credits will be subtracted, along with other additions/subtractions listed on your summary (these are your non-refundable credits). What is left is listed as your Total Tax on this summary. If it has been reduced to zero, then any additional amounts you paid, such as employer withholding on your W-2, and your refundable credits will all be refunded to you.