The “SNMP Community string” is like a user id or password that allows access
to a router's or other device's statistics. Community string is sent along with
all SNMP requests. If the community string is correct, the device responds with
the requested information. If the community string is incorrect, the device
simply discards the request and does not respond.

Note: SNMP Community strings are used only by devices which support SNMPv1 and
SNMPv2c protocol. SNMPv3 uses username/password authentication, along with an
encryption key.

By convention, most SNMPv1-v2c equipment ships from the factory with read-only
community string set to "public". It is standard practice for network managers
to change all the community strings to customized values in the device setup.