The Big Data in Real World team explains:
Let’s say you have a consumer group which has 3 consumers at the moment consuming messages from a topic. Assume that you had to shut down all 3 consumers in the consumer group for some reason. Now when you restart the consumers in the consumer group, how does the consumers know from which offset they should read from the topic to avoid reading the same messages all over again which were already read before the consumers went down?
Read on for the answer.