Community Strings were introduced in SNMP v2c and allow for a minimal level of security as well as allowing the administrator to group hosts into identifiable communities of hosts. Think of a community string as a kind of a not-so-secret password for SNMP communication. By default, all computers come configured with the community string "public".
On most SNMP v2c compatible systems there will be a read-only community string and a read/write community string. Provided the object being polled can be written to, the read/write community is needed. To restrict access to the SNMP agent, the read-only password of "public" can be changed on most systems.