Unix network file sharing, mounting remote directories so they appear as ordinary local paths. It is the standard way to share storage between Linux and BSD machines, and it underpins a great deal of cluster and virtualisation storage.
Traditional NFS trusts the client to state who the user is. On an untrusted network that is no protection at all. Keep it internal and use Kerberos where it matters.
NFS exposed beyond a trusted network effectively trusts strangers to declare their own user IDs. Restrict it to internal addresses, and consider Kerberos where the data matters.
The port scanner will tell you whether 2049 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.