Hi,
Like the "new" info bubbles that are present in the forum, you can now add tags to a thread's subject that will be enclosed in color filled bubbles of their own.
Clicking on each of these bubbles will activate the search function for that tag.
Note that the bubbles will not appear of the threaded messages, only on forum indexes, search results, and on the breadcrumb header of any post.
EDIT: Try not to use special chars or white space in the tags - search will not work properly with those.
Cheers