What does the "update" element do in Magento layout XML?

The <update> basically pulls in another handle.

Assume you have this:

<layout>
   <foo>
      <reference name="header">
          <block type="cms/block" name="some_block" as="someBlock">
              <action method="setBlockId"><block_id>some_block</block_id></action>
          </block>
      </reference>
      <reference name="left">
          <block type="cms/block" name="some_totally_different_block" as="someTotallyDifferentBlock">
              <action method="setBlockId"><block_id>some_totally_different_block</block_id></action>
          </block>
      </reference>
   </foo>
   <bar>
      <update handle="foo" /> 
      <reference name="header">
          <block type="cms/block" name="some_other_block" as="someOtherBlock">
              <action method="setBlockId"><block_id>some_other_block</block_id></action>
          </block>
      </reference>
   </bar>
</layout>

The resulting XML for bar would be:

<layout>
   <bar>
      <reference name="header">
          <!-- Start of part pulled in from foo -->
          <block type="cms/block" name="some_block" as="someBlock">
              <action method="setBlockId"><block_id>some_block</block_id></action>
          </block>
          <!-- End of part pulled in from foo -->
          <block type="cms/block" name="some_other_block" as="someOtherBlock">
              <action method="setBlockId"><block_id>some_other_block</block_id></action>
          </block>
      </reference>
      <!-- Start of part pulled in from foo -->
      <reference name="left">
          <block type="cms/block" name="some_totally_different_block" as="someTotallyDifferentBlock">
              <action method="setBlockId"><block_id>some_totally_different_block</block_id></action>
          </block>
      </reference>
      <!-- End of part pulled in from foo -->
   </bar>
</layout>

tl;dr: The update handle is basically a "merge this layout with my current layout".

This handle is used for merging existing layout handles to your current layout. In your example <update handle="editor"/> will add to the <adminhtml_catalog_product_edit> following content:

<editor>
    <reference name="head">
        <action method="setCanLoadExtJs"><flag>1</flag></action>
        <action method="addJs"><script>mage/adminhtml/variables.js</script></action>
        <action method="addJs"><script>mage/adminhtml/wysiwyg/widget.js</script></action>
        <action method="addJs"><script>lib/flex.js</script></action>
        <action method="addJs"><script>lib/FABridge.js</script></action>
        <action method="addJs"><script>mage/adminhtml/flexuploader.js</script></action>
        <action method="addJs"><script>mage/adminhtml/browser.js</script></action>
        <action method="addJs"><script>prototype/window.js</script></action>
        <action method="addItem"><type>js_css</type><name>prototype/windows/themes/default.css</name></action>
        <action method="addItem"><type>js_css</type><name>prototype/windows/themes/magento.css</name></action>
    </reference>
</editor>

("editor" handle is defined in app/design/adminhtml/default/default/layout/main.xml)