"Back door" conversion is an unofficial term for a loophole in the law that is used by people who earn too much to do a direct contribution to a Roth IRA. If you earn less than the income limit, then you will never need to use the "back door" loophole. Just make a direct contribution. (I'm assuming you're below the lower end of the limited range and can make a full contribution. If you're in the phase out range, a back door process might be better because it allows you to do the full amount.)
If you rollover 401K money to a Roth IRA, then that's a normal fully taxable conversion. There's no penalty. Just ordinary income tax, which might be more or less than 20%. It depends on how much you earn from other sources and how much is in the 401K. If you do the rollover in the year after you stop working and you have no other income, then some of the rollover will be taxed at 0%, some at 10%, some at 12%, some at 20%, etc, as you fill up each income bracket. When you get to that point, look at your financial situation and other income and consider whether it's better to rollover to a tIRA and then do one or more partial conversions to your Roth.