What does 'consumer group' mean, and why does adding more consumer instances only help scaling up to a point?

A consumer group is a set of consumer instances that jointly consume a topic, with each partition assigned to exactly one consumer in the group at a time. Scaling helps only up to the number of partitions — once you have as many consumer instances as partitions, adding more consumers leaves them idle, since a partition can't be split further across two consumers in the same group.