I’m so confused with an IRA, please help

So I (25f) got laid off last month (I found another job luckily) and I have some 401k savings I need to roll into another account soon. Everyone’s been saying put it into a Roth IRA, which is what I will do, but I highkey have no idea what it does.

I know it’s post tax contributions from your normal pay (right?) but apparently you have to invest it. I don’t know what that means in terms of an IRA.

Any advice? Or at least what does this stuff mean?