In general, there are 3 types of money that can be contributed to a 401(k) plan: Pre-Tax (traditional), Roth, and After-Tax. Pre-Tax gives a tax advantage by lowering taxable income in the year contributed, Roth provides a tax advantage by allowing the contribution basis to not be taxed again, and potentially tax-free earnings if some conditions are met.
At its face value, After-Tax contributions do not serve any tax advantages. If left sitting in an account, you will never be double-taxed on the contributions (basis), but any investment earnings on that amount are taxable when distributed. As a result, it is not as commonly found as an option in 401(k) plans. Some plans will allow for participants to continue to contribute beyond the IRS 402(g) limit that determines how much Pre-Tax and Roth can be contributed in a calendar year. Others may allow for it to be assigned a percentage or dollar amount just like other contribution types, but some plans do not match this source. Another piece to consider with After-Tax contributions is that many plans allow for in-service distribution of these funds. Check with your 401(k) plan provider to see what options your plan provides.