Since we are talking about a 401(k), there are two things that can be happening.
1. You can contribute money to a designated Roth option within the 401(k) plan. This is non-deductible for now, but won't be taxed when you withdraw it in retirement. It is only reported on your W-2 in box 12 with code AA.
2. You can convert money from a pre-tax 401(k) account to a designated Roth account. When you convert money from pre-tax to Roth, you must pay income tax on the amount converted. And the conversion will be reported on a 1099-R form that you get from the plan custodian.
It sounds like you contributed to the Roth option. Did you also convert from pre-tax to Roth? That is not clear from your question.
