The longer version
A custom domain is a DNS setting. The club owns a domain, adds a record that points a hostname, usually the main site or a subdomain such as book, at the booking software, and the software answers on that name with a valid TLS certificate.
It matters for trust and for search. Players type an address they know, links shared in a group chat carry the club’s name, and the pages rank under the club’s domain rather than a shared one. It also makes switching vendors later far easier, because the address stays.
Common problems are a record added at the wrong level, a certificate that never gets issued, and emails still arriving from the vendor’s address. Check all three the day it goes live: the site, the padlock, and the sender name on a test confirmation.