A Listener is an open network socket which clients use to communicate with Kafka. Listeners are configured with a host (IPs or hostnames), port and a protocol mapping. Protocol mappings allow Kafka to accept clients on different host:ports with different security configurations. For example you may want different security configurations for interbroker communications than you do for external client connections.
@rmoff’s blog post on the topic goes into the nitty gritty: https://www.confluent.io/blog/kafka-listeners-explained/