system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is a 401k and how does it work?

I understand that it's some sort of retirement savings account, but beyond that i don't really know much. Employer matching, how people can "lose" theirs and all that.
gpt
401k refers to the legal code that allows this type of savings account to exist. 

It allows you to set aside a certain amount of your income, each pay period, to go into a retirement savings account, tax ~~free~~ deferred (you pay the tax at retirement when you withdraw - the presumption is that you won't have a job at retirement, so you'll have a lower tax bracket). 

Your employer, if they don't suck, may match part of your contribution - these matched contributions vary from as low as like 3% of 25% of your contribution to 10% of your entire contribution. 

An individual investor doesn't have much power, but with a 401k, everyone in the company usually gets lumped into a big savings plan together, giving them more power. 

However, that savings plan is an investment plan. It invests in bonds, stocks, etc. 

If the stock market takes a bath, your 401k can lose a lot of money, include the contributions of you and your employer. 

If you take money out of a 401k early, you owe extra taxes on it (since it was tax free). But if you can let it sit until you're like 65, then you get the money ~~tax free~~ without a penalty. 

Hope that helps.