In this video we cover a new Drupal 7 feature which lets us create suggestions for theme functions, in addition to suggestions for templates. We'll use this to modify the tags displaying on a node in our site.
In the example:
$item['#options]['attributes'] += $variables['item_attributes_array'][$delta]; += is a shortcut for array_merge. So it is ensuring the contents of $variables['item_attributes_array'][$delta] are included in $item['#options]['attributes'] so that any class, rdf info or other html attributes are appropriate applied to the tag links. It's possible for any number of modules to set HTML attributes on the tags field in the theme we need to make sure that they get printed out appropriately.