A probe is a device dedicated to the task of network monitoring. Because probes are dedicated to this task, they are not burdened with other responsibilities that could affect the performance of its monitoring capability. It is akin to a remote controlled network analyzer.
The device is likely to support RMON, which allows detailed information to be collected via SNMP. Probes are often initially deployed on WAN links, as they are often a crucial resource to the backbone, and issues there can affect many users.