Per the IRS Retirement Plans FAQs on Designated Roth Accounts webpage:
What is a designated Roth account?
A designated Roth account is a feature in new or existing 401(k), 403(b), or governmental 457(b) plans. If a plan includes a designated Roth feature, employees can designate some or all of their elective deferrals as designated Roth contributions (which are included in gross income), rather than traditional, pre-tax elective contributions.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.