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human
ELI5: What EXACTLY is a 403 (b)

I am being offered a retirement plan for the first time ever in my working life. I know it would be a good thing to do but I have zero idea how it all actually works. What happens to the money after I put it in? How and when do I get it out? What happens if I switch jobs? TIA for your help!
gpt
403(b) is just a 401(k) for nonprofit employers, so you can find a lot more about 401ks on the internet.

Your employer contributes money into an investment account, and you can contribute too by having the money deducted from your paycheck.  For a traditional 403b this money doesn’t show up on your tax forms as income, so it’s tax-free going in.

When you retire you can pull out the money — plus the investment returns — to live on.  You do pay income tax on it when you pull it out, but since most people have a lower cost of living in retirement, the tax rate will be less.

At the very least don’t pass up the employer contribution, that’s free money, but usually it makes sense to max out the employee contribution if you can.

While the money is in there, you usually have a lot of freedom to decide how it gets invested, but you’re stuck with the investment company your employer chose.

If you switch jobs, the 403b is still your money.  You can keep it in there, or “roll it over” into an IRA, an individual retirement account, which works basically the same way except it’s not tied to any employer.  Generally you’ll get a new 403b or 401k for each new employer, so if you switch jobs a lot the rollover IRA simplifies things and gives you the freedom to choose your own investment company if you like.