A simple in-memory cache with no authentication by design — it was built for a trusted network behind an application, where the only thing that can reach it is your own code. It stores key-value pairs and forgets them when restarted.
Its UDP interface enabled record-breaking amplification attacks, because a tiny request could return an enormous response. Disable UDP, bind to localhost, and never let it face the internet.
Disable the UDP interface first, since that is the amplification vector, then bind to localhost. There is no configuration in which public Memcached is appropriate.
The port scanner will tell you whether 11211 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.