Объясните selectedOption,пожалуйста?

<select>
  <option value="Rock">Рок</option>
  <option value="Blues" selected>Блюз</option>
</select>

<script>
  var select = document.body.children[0];

  var selectedOption = select.options[select.selectedIndex]; //
  alert( selectedOption.value  + " - " + selectedOption.text );

</script>