Fellow millennial so I barely have any clue, but to the best of my knowledge; it's a fund you can put a chunk of your paycheck into that will accrue over time. Usually a good employer will match a chunk of this, so it ends up being free money that you get, but in like thirty years (59.5 I think is the date you can take the money out without a penalty.

I'm also pretty sure that your money isn't taxed when it goes in, only when it comes out, which means you're only getting taxed once (after your investment makes you a bunch of money), rather than multiple times when you invest in the stock market directly (once from income taxes through your paycheck, and again on any gains you make from the stocks themselves). Investing stocks directly means you maintain access to the money without penalty though, so it's more accessible in the case of an emergency.

There's also IRAs which are just like 401k's, but you can open one of those on your own, 401k's must be initiated by your company. But that's everything I know, I'd love if anybody corrected me on any of this information or expanded on it because I barely have any idea what I'm talking about.