A credit applies against your tax liability. So if you have a tax credit for $100, your tax liability is reduced by $100. Credits generally do carry over to future years to the extent unusable in the current year.

Whether a credit is preferable to a deduction depends on the taxpayer's tax situation, including whether they would be able to make use of carried-over credits in future years.

Edit: in a previous comment I said R&D credits aren't available if you don't have positive income. That's not strictly true. R&D credits only apply against actual tax liabilities, but if you don't have any taxable income the unused credits would carry over into a future year for potential use.

It's just like if you hire a builder to make you a rental property. You can't deduct 100% of the costs of that house in the year it's built.