Template fields are variables that will change for each applicant who receives the message. Using these template fields allows you to send one message to multiple applicants, yet still make each message seem personalized to the recipient because you include their first name, for example.
Templates available are listed in the "insert template" dropdown in your messages window:
Messaging applicants who have already submitted (from the Messages page)
Messaging applicants who are still in progress (from the Submission table)
Automated scheduling messages (from Applicant Notifications)
In addition, if your organization is using Admin Only fields, these will turn into template fields so you can send mass messages using the answers to these admin questions.