What is the 'add tags' feature?

You can add tags at any level to keep your projects, board or tasks trackable and organized. For adding tags you simply have to type the tag name or simply select it from the autosuggest list. If you wish to categorize your projects based on certain keywords you can use tags for projects. 

For adding tags to tasks, open the task and scroll to the bottom. You will find an option to add tags.