Name resolution: turning example.com into an IP address. Queries normally use UDP, falling back to TCP for large responses and for zone transfers.
An open recursive resolver is a favourite amplification tool for denial-of-service attacks, because a small query produces a large answer. A resolver should answer only its own network.
Open 53 is correct on a name server. On anything else, check whether it is a recursive resolver answering the whole internet, which makes you an amplification source. It should answer only your own network.
The port scanner will tell you whether 53 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.