[HELP] What is Read/write community string?

I have a problem to add new nodes, I started to add manually the nodes for SAM, in "Define node" i choose "polling method" - "most devices: SNMP and ICMP - Recommended", SNMP v2, port 161, community string: public but i don't know what is read/write community string? , where i can find it and what can i do in this situation because I couldn't add node, always send message: "Node does not respond with the supplied read/write community string",

please help me to figure out what is happening?

Thanks all