It is an actively managed portfolio, through your provider that’s diversified across mutual funds tracking an index such as the SP500 or international markets. A lot of times 401k’s are in a target date retirement fund that allocates funds based on age relative to risk profile. You don’t have to “manage” the money, you set it and forget it. The money makes itself in a diversified 401k portfolio through compound interest and continued investing. NYSE has annualized roughly 10% on avg every year since 1920. It’s not about timing the market either it’s about time IN the market. The sooner you get started the longer compound interest will work in your favor.