The idea
DNS supplies records associated with names. A records commonly map a hostname to an IPv4 address, AAAA records to IPv6 addresses and CNAME records alias one name to another. A resolver can cache answers according to their TTL. DNS identifies how to locate a service; it does not guarantee that the service is running or that its content is correct.
Worked example
A fictional shop.example has an A record pointing to the documentation address 192.0.2.10. A www hostname can be an alias to another name. After an address change, a resolver with an unexpired cached answer may continue returning the old address. The example domain and address are for explanation, not a service to probe.
Try it
Draw a browser, resolver and authoritative DNS service. Label an A record, a CNAME and a TTL. Explain why two clients might temporarily see different addresses after a record change. Write why editing DNS would not repair an application error on an already reachable server.