Tags are a lightweight, flexible, and powerful feature that enables you to categorize conversations and keep track of certain topics in the mailbox.
Several tags can be added at once.
Under “Manage » Tags” you can view and edit existing tags:
You can click on a tag and see all the tagged conversations:
In “Settings » General” you can allow users to manage tags (i.e. access to https://your-domain.com/tags page):