master vs slave (clustering)

Hi. please some body help me. I don't know what is difference Master and Slave.

I do clustering. I have 4 node and use 2 master and 2 slave. I use this link
https://raymii.org/s/tutorials/Set_up_a_federated_XMPP_Chat_Network_with...

all nodes have same /etc/hosts
192.168.120.70 70_slave
192.168.120.69 69_slave
192.168.120.68 talk ## is domain
192.168.120.67 67_master
192.168.120.68 68_master

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