Simple Network Management Protocol. It's the most complicated thing with the word simple in it I've ever seen.
Bascially it's a standard, like TCP/IP, except it allows you to communicate with other network devices that "speak" SNMP. You can monitor and even configure devices via SNMP.
Simple, no. Effective, yes. There are some pretty large texts written on it, so a simple explanation of this protocol is not warranted in this forum. 🙂