It selects elements with the class `class` that also have the class `class`. The example might be a little abstract but it could be something like `.message.highlight`, which selects all elements with the class `message` that also have the class `highlight`.

Classes are for classification, IDs are identification. For styling you almost never need identifiers because that is not what styling is about.

  <div class="message highlight">
  .message.highlight {}

  <div id="highlightMessage">
  #highlightMessage {}