In a network of any size, it is only a matter of time before an issue arises due to intentional or unintentional circumstances. The root cause of an issue could be hardware failures, software issues, human errors, or just about any other environmental factor that causes loss of network or data. Network monitoring is a practice in which an administrator checks the pulse of the network components in a network environment.
There is no system to monitor everything. A good monitoring system is usually put together with various tools and some types of notification options to send alerts automatically. The Proxmox cluster is a sum of switches, network cables, physical nodes acting as hosts, and virtual machines. A monitoring system should be able to monitor all of these components and automatically send notifications via a medium, such as an email or SMS, to responsible parties. There are wide ranges of network monitoring tools available today, such as Icinga, Zabbix...