title: What does PLAINTEXT keyword means in Kafka configuration?
body: <p>In <a href="https://kafka.apache.org/documentation/" rel="noreferrer">Apache Kafka Documentation</a> they use keyword <em>PLAINTEXT</em> as a protocol in many default settings:</p>
  
  <p>e.g. 
  <code>listeners=PLAINTEXT://broker1:9091,SSL://broker1:9092</code></p>
  
  <p>Does it represent just a default non-SSL protocol and if so, why they use <em>PLAINTEXT</em> keyword for that?</p>