This feature allows you to show tags on the Agile board and filter issues by just clicking over the tag's name. To use it, you need to install both Agile and Tags plugins.
To show tags, go to Agile board options and check Tags.
Tags will appear on the Agile board and you will be able to use quick filtering by tags.
By clicking on a tag's name, you will get a list of all issues marked with it.
You can set user permissions at Administration > Roles and Permissions (scroll down to Forums and/or Issue tracking - if included in the list).
Since version 2.0.6 of the Tags plugin, the tags are also available on the Version planning board of the Agile plugin.