IPv6 addresses are 128 bits long, four times the length of an IPv4 address, which is where the name "quad-A" comes from. A resolver that wants to reach a host over IPv6 asks for its AAAA record and receives an address such as 2001:db8::10.
Publishing an AAAA record next to the A record makes a host dual-stack. Modern clients try both and use whichever connects faster, so a broken IPv6 path, with an AAAA record pointing at a server that does not actually answer over IPv6, causes slow or failing connections for part of your visitors. Only publish AAAA records for addresses that really serve the site.
If a domain has no AAAA record, that is not an error: it simply means the service is reachable over IPv4 only.
Example
example.com. 300 IN AAAA 2001:db8::10