As an agent when you are working on tickets or accessing articles, characterizing them by adding a tag would help to track and segregate them with respect to issues or requests. 

Please navigate to Admin > Tags > Add new tag which would be available to add in the tags filter within the tickets details page. Once the tag is created, you can associate tags with Tickets, Contact, and Articles. Tags can be managed from the same window as well.

You can also merge two tags by editing one of the tags and making the name of one tag, the same as the other.

A tag can also be added from inside the Ticket's Details page and the existing tags will also be displayed as shown in the images below.