DNS can map a server-hostname an addressable machine (by IP address) on the Internet.
A server-hostname may be comprised of a domain name + optional subdomain. (e.g. example.com or www.example.com).
In order to do this, the DNS-Resolver of a computer/client may issue multiple queries, recursively, until the machine address is resolved.
As a lot of requests/queries are required to resolve a single name to an IP-address.
One of the possible optimizations is for the local resolvers/dns-servers to “cache” results it retrieved before and store them for a specified amount of time.
(This is called TTL…Time To Live)
Important: Due to this mechanism DNS-Updates will take some time to “propagate”. Changes may take up to 24 hours to take full effect.