DNS
A practical DevOps definition, with an example you can use.
The Domain Name System: a distributed system that resolves names into records such as IP addresses.
An example you can picture
Your browser looks up the address for an application hostname before connecting to its server. A cached record can remain in use until its time to live expires.
The distinction that matters
A DNS lookup succeeding does not prove the application is healthy. Name resolution, network connectivity, TLS, and application responses are separate checks.
Put it into practice
Look up a public hostname with dig or nslookup. Identify the record type and returned value. Then explain which additional checks you would make if name resolution worked but the page would not load.
Connect the ideas: load balancer and availability. For the bigger picture, read the related learning guide.
Reference
Read the primary documentation for the underlying concept and further detail.