Tags let you label messages across folders, and will allow you to organize and find messages more easily.
To add a tag, open an email, select [New Tag] from the Tags field near the top, enter a tag name, and click “Add tag”. You can now sort messages by your tags in the message list.
To delete a tag from an email, open the message and click the little [X] next to the tag name in the header area of the message.
You manage your tags by clicking Tags in the left pane in Webmail. The Tags screen lets you add and delete tags, and get an overview of the messages that are already tagged.