In short, Kafka is a horizontally scalable streaming platform. In other words, Kafka is a message broker which can be run on multiple servers as a cluster. Different data streams are called topics. Producers can place messages on a topic whereas consumers can subscribe to topics. Topics can be configured for single- and multiple delivery of messages. Consumers can be grouped in so called consumer-groups, which makes it possible for multiple consumers to act as one when it comes to single-delivery.
But don’t take my word for it. There’s a lot more to Kafka than I can get into in this post and the original documentation is much clearer, so check out the documentation at https://kafka.apache.org/.