Where Windows clients ask which dynamic port a given RPC service is on. Fundamental to Active Directory, WMI and a lot of Windows management traffic.
Should never be reachable from the internet. It has been involved in a long line of severe Windows vulnerabilities and offers an attacker a map of available services.
Port 135 reachable from the internet is a firewall failure. It should be blocked at the edge without exception and is only ever appropriate inside a trusted network.
The port scanner will tell you whether 135 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.