A domain name is essentially the main element of a web address, for example u3a.org.uk. Some u3as will have registered a domain name of their choice, for example Hammersmith and Fulham have registered handfu3a.org.uk. Along with your domain name your u3a may be subscribed to hosting and email services.
Check that your u3a has access to the management facilities of your domain. This will be with an Internet Service Provider (ISP) or domain name registrar. It is far from unusual to have your domain set-up by a member who has moved on without making transfer arrangements.
If you are migrating to SiteWorks and your domain points to, or redirects to, your Site Builder site then it will need updating through your ISP/registrar to point to your SiteWorks site when it goes live. If you don’t do this then redirection should still happen, but it will be through a redirection chain – first to Site Builder and then to SiteWorks. This chain will be broken when Site Builder is eventually decommissioned so plan to do this when you migrate.