SNMP or Simple Network Management Protocol (Wikipedia: SNMP) is a protocol that allows devices to be managed remotely over the network. SNMP provides a wide array of of capabilities for monitoring and configuration.
PeakHour can use SNMP to provide information about each interface your network device(s) and the amount of traffic passing through it.
SNMPv1 has been around since 1988 and is widely supported on many many times, makes and models of network devices. This is especially true of enterprise networking hardware where it is considered the gold standard.
In residential network devices, SNMP is sometimes seen as a 'high end' feature and therefore only supported in some routers.
The PeakHour Compatibility Check tool can determine whether or not a device supports SNMP and whether it can be monitored by PeakHour. We highly recommend you try this tool first to ensure the device(s) you want to monitor are compatible. Even if a device claims to support SNMP, it may not provide all of the necessary information that's required.