If you have domains with another domain provider, you can use them on your o2switch hosting account. They are talking about pointing a domain to o2switch.
You have two options if you want to use one of the domains you have elsewhere: you can transfer the domain entirely so that o2switch recovers the complete management of the domain. Complete management means administrative and technical management. The domain changes registry. or, you can keep the domain with the provider where it is currently located and change the DNS to point the domain to your o2switch web hosting. They are talking about a technical change in this case. The administrative part does not change. After pointing the domain to o2switch, with either of these methods, the next step is to configure the domain on your hosting using the additional domains tool.