What is a partition in Kafka and why are they important?

A partition is the fundamental unit of parallelism in Kafka. Each topic is split into N partitions, and each partition is an ordered, immutable sequence of records stored on disk. Partitions enable horizontal scaling: different partitions can be stored on different brokers, and different consumers in a group can read different partitions simultaneously. The number of partitions determines the maximum consumer parallelism — you can't have more active consumers in a group than there are partitions.

Expected key concepts: parallelism, ordered, immutable, horizontal scaling, brokers, consumer parallelism, maximum consumers