Insecure network services can be exposed if ports are open on a webserver which are not absolutely necessary. An attacker can scan the server for open ports and find possibly vulnerable services on the server. If insecure network services are detected by the attacker it would be possible to exploit vulnerabilities in the service like buffer overflows or denial of service. This can lead to a compromised or not fully working webserver.
To prevent insecure network services to be exposed it is important to identify all services that are necessary and open only ports which are used by these services. All other ports should be closed by setting up a firewall. Furthermore if a service is not needed it should be uninstalled in order to keep the attack surface as small as possible.