The service that WordPress.com provides called ‘Domain Mapping’ is essentially just a process to redirect the free subdomain that they provide yourblog.wordpress.com to a new domain that you might have or register (purchase).
A redirect is sort of like an official (Google bot accepted) detour sign. It says our location has moved for our home page, and every other post, page, category, tag, archive, image, everything. It has moved from our old sub domain location to our new domain location.
Anyone that happens to have one of your old sub domain hyperlinks and clicks on it, will be automatically sent to the new location of that same item in the new domain.
If you plan to migrate your site from WordPress.com to some other service. We highly recommend that you have the Domain Mapped by WordPress.com first. This is an important first step. This should be done before you export your content from wordpress.com to a self hosted server or setup somewhere else.
More information on how the Domain Mapping steps work are here.
← What does Domain Mapping actually entail and involve? Wha is included in the actual domain mapping to prepare the wordpress.com site to be completely converted to another domain, say a .org, .com or some other domain?