It is one of the key CONCEPTS in Kafka. The replication ensures data is safe and secure even in case of system failure. The Kafka STORES the published messages which are broadcasted to subscribed systems but what if Kafka server goes down, will published messages be available when the system goes down? The answer is yes and it is all possible due to replication behaviour where messages are replicated across multiple servers or nodes. There could be multiple reasons for system failure like program failure, system error or frequent software upgrades. The replication safeguard published data in case of any such failures. The fault-tolerant behaviour is one of the key reasons why Kafka has become a MARKET leader in a very short period of time. ISR which stands for In sync REPLICAS ensures sync between the leader and follower systems. If replicas are not in sync with ISR then it points that follower systems are lagging behind leaders and not catching up with leader activities. Please refer to the below DIAGRAM which will make your understanding more crystal clear :