I have my domain codonomics.com registered with bigrock.in.
Now when I do a nslookup for my registered domain against BigRock's DNS server, what I get would be an authoritative answer, because this server holds the original source files (the A, AAA and other records) of my domain.
If I were to do a nslookup for my registered domain against any other name-server, that would act as an intermediary to get the details from the its sources. And when this happens, it communicates in its response as non-authoritative answer in nslookup.
Check this out in action in your terminal like shown in the blog banner image for this domain. And do play with other domain names as well if you know their source DNS servers.
Understanding this is perhaps the first step to doing more on the networking side of architecting your infrastructure. For instance, understanding How to Set Up DNS Resolution Between On-Premises Networks and AWS Using AWS Directory Service and Amazon Route 53, mandates your basic understanding of this concept of authoritative and non-authoritative DNS.