A quick Google search will tell you that a watcher is "a person who observes something attentively or regularly".
The 'watchers' feature is best described as an 'ongoing carbon copy' of ticket updates to people that need to be kept informed of changes to the ticket's (or customer's) status.
This term is widely used in ticketing systems for software development that track bugs however there are many other reasons to add watchers to your tickets:
How to add watchers to a ticket:
Q: Can watchers respond to the ticket-email updates they receive? If they do, what happens?A: Yes, If a watcher responds to a ticket update via email their comments are marked private and will be invisible to the customer since they are not the ticket assignee.
Q: Can operators remove themselves as watchers?A: Yes they can, in fact any operator can remove a ticket watcher in the same manner that a watcher is added (explained in steps 4 & 5 above).