A naked domain (also known as a root domain or zone apex) is a domain name (like indieweb.org) that doesn’t start with “www.” or any other subdomain and is preferred for IndieWeb sites as identifying yourself starting with W-W-W gets tiresome quickly. Sites that use naked domains can offer naked URLs which are shorter than those prefixed with “www.” and thus better URL design.
Apex technical details
The zone apex is the part of DNS that contains an the SOA and NS records for a DNS zone. If you have a site www.example.com, then then the apex is example.com. The www. portion might be a dedicated host (or load balancer) for serving Web traffic (as opposed to other Internet services. The DNS specification forbids serving a naked domain with a CNAME at the apex. Some DNS providers, such as CloudFlare, give the appearance of support for CNAME at the apex by simulating it by "CNAME flattening", which accepts a CNAME in the configuration panel as if it were allowed, but silently serves up an A record with the correct IP for DNS queries. Some providers might also call this an ALIAS record, which is not a real DNS record, but the name for the placeholder which uses the same strategy.