The standalone rsync daemon, offering named modules of files for synchronisation without tunnelling over SSH. It is common on public mirrors for Linux distributions, and inside organisations for backup jobs that predate anyone thinking about encryption.
Frequently deployed with no authentication and world-readable modules. Publicly exposed rsync servers routinely leak entire backup sets. Run rsync over SSH instead.
An open rsync daemon deserves immediate attention. Check whether its modules require authentication, because the common misconfiguration exposes entire backup archives to anonymous download.
The port scanner will tell you whether 873 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.