Mass Reply is used for sending same resolution/reply/email to multiple users. It helps staff to save their time for sending same reply to multiple tickets. It can be done from Staff CP > Tickets > Manage Tickets : select multiple tickets and click Mass Reply.
Once you click the reply button, you'll get a text box in which you can type your response.
This feature comes handy when there is a scheduled maintenance that needs to be carried out, and an email needs to be sent to multiple customers. This or similar type of mailers can be sent to multiple customers by using the "Mass reply" feature.