A credit is literally cash (assuming you have enough liability to cover it).

A deduction reduces the amount of income thats taxed. So for example if you're in a 25% marginal tax bracket, deductions are worth 25 cents on the dollar.

A credit reduces the amount of tax you owe, dollar for dollar. In addition, some credits are refundable, meaning that they're allowed to make your tax liability negative for the year, so you get benefit from them even if the credit is more than the taxes you would otherwise owe.

And to be clear, when I talk about the "tax you owe", I mean the total amount of money you need to have paid the IRS by April 15th, including payroll withholding, estimated tax payments, and so on, and not the size of check you need to mail to the IRS in early April.