title: What does "1", "2", "3" mean in protobuf?
body: <pre><code>package tutorial;
  
  message Person {
    required string name = 1;
    required int32 id = 2;
    optional string email = 3;
  }
  </code></pre>
  
  <p>What does "1", "2", "3" mean ?  </p>