A Roth conversion allows you to convert a pre-tax retirement funds into an after-tax, or “Roth” funds. This simply means you pay taxes on your contributions at the time of the conversion instead of later, when you withdrawal at retirement. In either case, your investments will continue to grow tax-free.
In some cases, you can do a Roth conversion as part of your rollover, if your 401(k) plan and your destination IRA provider allows for it.
If you're not able to do the conversion at the same time of your rollover, you can always do the conversion after your funds are in the IRA!