"Mega backdoor Roth":
Step 1) Contribute after-tax (not Roth) money to your 401(k).
Step 2) Roll it over to Roth IRA or 401(k).
"Backdoor Roth"
Step 1) Contribute non-deductible money to a traditional IRA.
Step 2) Roll it over to a Roth IRA.
The benefits to each of these are simple: you get to put money into a Roth IRA through this indirect method that you wouldn't be able to otherwise. This money will then grow tax-free.
There are some caveats. For the non-mega backdoor Roth to work, you need to not have any pre-tax money in any traditional IRAs. For the mega backdoor Roth to work, your employer needs to allow after-tax contributions above and beyond the standard $18k limit, and they need to allow in-service withdrawals of these contributions.