What is a Roth IRA?

A Roth IRA like a traditional IRA is an Individual Retirement Account. But with a conventional IRA, contributions are made pre-tax, and with a Roth IRA, you make your contribution after tax. So a traditional IRA will lower your current tax-base. But a Roth IRA offers tax-free withdrawls at retirement. That can have huge advantages if the tax rate goes up over the years.
You can also invest in a Roth IRA at the same time you are making investments in a 401k. If you have a lot of money to invest, you can contribute the maximum to your 401k, and then contribute to a Roth IRA. Then at retirement, you will have money in your 401k that will be taxed upon withdrawal. But you will also have tax-free money in your Roth IRA. This way, you are diversified in a way tax-wise between your two accounts.
You can continue to contribute money to your Roth IRA after you reach age 70 1/2. You can leave money in there for rest of your life, potentially leaving tax-free money to your heirs.