Following a ticket BCCs your agent email address onto any email correspondence from the ticket. This was originally created to ensure that when any ticket is escalated from someone less experienced to someone more experienced, the less experienced individual then learns how to resolve the ticket themselves in the future by being BCC'd into the resolution emails.
Following a Ticket
You can follow any ticket you wish to in Halo, provided you have added the "Followers" field onto the ticket type in question and you have permission to view the ticket in question.
To follow a ticket, press the Eye icon in the top right of the inner ticket window and clicking 'Follow this Ticket'. You can also view other followers of the ticket here too:
Clicking 'Add Follower' will open a new menu allowing you to search users, agents and site's users and add them to the followers list.