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What is a DNS A record?
The A record points a hostname at an IPv4 address. It is the most frequently used record in a DNS zone.
An A record maps a name such as www.example.com to a single IPv4 address. When a resolver asks for the address of that name, this is the record that answers.
A name can carry several A records. Resolvers then receive all of them and pick one, which spreads requests across the addresses but is not a health check — a dead address stays in the answer until you remove it.
The TTL decides how long a resolver may reuse the answer. Before you move a service to a new address, lower the TTL well ahead of the change, then raise it again afterwards.
For IPv6 the equivalent is the AAAA record. A host that should be reachable over both protocols needs both, and both should resolve back to the same name in reverse DNS.