A Roth IRA is a retirement savings account that allows your money to grow tax-free. You fund a Roth with after-tax dollars, meaning you've already paid taxes on the money you put into it. In return for no up-front tax break, your money grows and grows tax free, and when you withdraw at retirement, you pay no taxes. That's right - every penny goes straight in your pocket.
There are a few other benefits too. For more see How is a Roth IRA different from a regular IRA?