What is dig

Dig is a short from Domain Information Groper. It perform DNS lookups and displays the answers that are returned from the name server(s) that were queried.This tool is an alternative DNS lookup tool, nslookup. Unless it is told to query a specific name server, dig will try each of the servers listed in /etc/resolv.conf

Full article here:
Some powerful options of Linux Dig command to Query DNS (LinOxide)
Related:
Domain Information Groper (DIG) — DNS Query Tool (Gaurav Kansal)