Polls network devices for status: interface counters, error rates, temperatures, uptime, disk usage. Almost every monitoring system speaks it, and almost every switch, router, printer and UPS answers it. Devices can also push unsolicited alerts, called traps, on port 162.
SNMP v1 and v2c authenticate with a community string sent in clear text, and a great many devices still ship with it set to "public". Exposed SNMP hands over a detailed inventory of your network. Use v3, and never expose it publicly.
An open SNMP port is worth acting on quickly. Check whether it still uses the default community string, because if it does, your entire network inventory is readable by anyone who asks.
The port scanner will tell you whether 161 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.