To give an example, assume your u3a is called Blueberry and browsers use the link (URL) u3asites.org.uk/blueberry to access Site Builder today. On SiteWorks it will be blueberry.u3asite.uk (both links without the https:// and optional www.). The name you choose can be different to that on Site Builder, you will be asked to decide at the site of the creation of your SiteWorks site but after that it cannot be changed so give it some thought.
This setup is different from Site Builder in that the site will be what is known as a subdomain on the Internet. A subdomain name helps with search engine indexing and will highlight your u3a or site name at the start of the URL rather than the end behind a ‘/’.