A and AAAA are the basic address records in DNS. A name can hold several A or AAAA records; clients pick one of the addresses, which gives you simple round-robin load distribution.
When a host has both types, most clients prefer IPv6 (Happy Eyeballs). An AAAA record pointing to an IPv6 address where nothing is listening causes delays or errors, even though IPv4 works fine.
Classic case after a server move: the A record was updated but the old AAAA record was forgotten, so part of your users still reach the old server.
Example
dig +short A cloud.example.com; dig +short AAAA cloud.example.com