A dollar-for-dollar reduction of the tax you owe.
A tax credit directly reduces your tax bill, making it more valuable than a deduction of the same size. Nonrefundable credits can reduce tax only to zero, while refundable credits can produce a refund beyond what you paid in. Credits exist for things like child care, education, and certain energy investments.
A $1,000 credit cuts your tax bill by the full $1,000, regardless of bracket.